i have to agree with the point of picking your strength and playing it constantly.
the more you play it the more you can hone it. the more you can understand it. the more you'll learn what works against it too. it'll end your losing streaks over time for sure if you're analyzing things correctly and adjusting. from those experiences you can then learn what makes other archetypes viable as you lose to them. for example: if you're playing aggro and keep losing to control, as your aggro play gets stronger and you start beating those decks, youll start understanding what makes those decks work. this means when you want to try something different you'll have a greater understanding of how to play that archetype right out of the gate. in turn this means you don't have to repeat the entire process to the same degree all over again. from this you can develop breadth of experience and knowledge. jumping around from archetype/deck to archetype/deck without mastering any really just makes you a sloppy player. it doesn't hone your skills or make you understand why something actually works or doesn't work. you're more likely to just get frustrated by losing, and you wont' necessarily have the skills yet to do more than that.
I think ultimately both options will improve you as a player. Focusing one at a time, or spreading it around - either way, playing more magic generally means getting better at magic. It probably comes down to player preference which works best for them and their learning style. For me, personally, I think I learn the most quickly when I'm trying new things, because there's more stuff to learn. If you picked up a new instrument every month, you'd be learning a ton of stuff constantly, you'd get better at being able to pick up instruments you weren't familiar with, and you'd learn a lot about music as a whole. But you probably wouldn't be first chair in any particular one. So it also depends on what your goals are. For me, I really don't care about mastering some specific archetype, because for me the exploration is a huge part of the fun of magic. Just being really good at playing one particular deck doesn't appeal to me, and that's why I've never seriously stuck with any constructed format at a competitive level, why I've built over a hundred commander decks, and why most of them usually only get played a couple times each. But if you prefer to find your lane and stick to it, then the other option might be better for you.
One thing I will say - in draft, there's a common problem with newer drafters (or even experienced ones who aren't as familiar with the format) where you can start to get a skewed perspective based on limited information. For example, you draft RB a couple times early on, do well, and now you're overvaluing red and black cards and end up forcing RB a lot more than you should. Or you draft aggro a few times, and now you're in the "always be attacking" mindset even when you ought to be on the defensive. Sticking with one mode of play I think can make it easier to form patterns in your play that you don't think about enough, because they're frequently correct for the kind of decks you've been playing. It can make you rigid in your thinking instead of looking at every angle. Not that it can't be overcome by critically analyzing your plays, but if I was trying to make someone good at magic, I'd be pushing them into new spaces so that they're constantly considering all possibilities during a game instead of falling into repetitive patterns.
I think a big part of why we're disagreeing is that, competitively, I focus primarily on limited, where obviously you don't have the luxury of specialization. You have to be ready to play whatever is open, whatever that may be. And the philosophy does affect how I play commander.
The main thing I would disagree with about what you've said is that I think you're looking at not just from constructed (which is obviously fair, given that we're talking about a constructed format) but a very high-level of play. Sure, some pro players favor certain types of decks over others, but any pro player worth their salt is good enough with the basics of play to pick up most random decks and play it decently, without even being familiar with the format.
That's just not the level most commander players are at. I know plenty of people that have basically picked up one deck because they liked the sound of it, and then just upgraded it occasionally and never tried anything else. When players are still at that really basic level, I think it really behooves them to try new things before they build up bad habits. For example, I've played limited against players who I've also played commander with, and one common mistake is that they'll play overly-defensively because they're used to commander combat where life totals are high, chances of salty retaliation are relevant, and playing defensively is generally the higher EV option - and also just the social norm. They're playing overly-defensively in commander too, of course - but they're not getting punished for it so they aren't aware of it. Getting into another format where combat is more effective can help shake up their habits a little, instead of autopiloting along with what they're used to and missing stuff. Then they can return to their norm with a more holistic understanding of the game - or maybe decide that they want to focus on some aspect that they weren't as familiar with before, because trying it out appealed to them.
I know when I first started playing commander, before I'd done anything competitively, I was very control-focused. And I still am, to a certain extent, but time spent playing limited and exploring other archetypes within commander made me more aware of what I like (and what I don't), and I've been able to use the lessons I've learned to all my decks - lessons that I wouldn't have gotten if I'd never left my safe control bunker.
Pro players may have preferences, but no successful player is married to one specific style of deck. If a pro player that likes control decks is going a GP in a meta where he thinks the best deck is aggro, he's not bringing a control deck. He's bringing an aggro deck. Even moreso for limited - if he prefers blue but white is what's open, then he's going into white and you can bet that he's prepared to play it.
My understanding is that, when doing specific tournament prep, pro teams will commonly play most meta decks against each other so they can decide for themselves where they think the meta is going to get an edge, but I'm sure that varies from team to team. At any rate, those players have almost certainly put in tons of hours playing some form of every archetype. Perhaps one is mastered more than the others, but they're going to be able to play every archetype at a much higher level than most players.
Can't speak for other pros, but I watch Kibler stream (hearthstone) all the time, and he switches from deck to deck and even format to format all the time. When he wants to hit legend, sure, he decides on the deck he wants to do it with - but he makes that decision based on having played many different decks so that he knows them all quite well and understands the meta inside and out. Sure, at a very high level it pays to be dedicated to a certain deck for a given constructed season, but that's so far above where most EDH players are at that it's hardly worth thinking about imo.
I think we still disagree. My opinion is that, ultimately, your goal should be to not have any "strengths". Not just to not focus on reanimator specifically, but not to focus on toolboxing more generally, or any other thing you might imagine. You should strive to do do EVERYTHING. There's no reason you can't be good at everything magic has to offer, and knowing more about every piece of it will make you a better overall player because you'll be able to apply lessons from one area to another, and to see enemy lines better and play around them.
Whenever someone asks me "what colors do you play?" or "what kinds of decks do you play?", that's a hint to me that they're either new or a weak player. Magic is not a game about finding your niche and burrowing in, that's how you stagnate. It's about constantly finding new territory to conquer.
I agree with Cranky here - if you want to win THE NEXT game, then stick with what you know...but unless you're going to a grand prix or something with an entry fee, imo you should try to become a better magic player holistically. Play every color, play different formats (try limited! we have cookies!), play different archetypes, borrow decks - everything you change up, your brain will absorb new ways of thinking. And one day you'll wake up and realize that you've gotten really good at the game.
Gotta agree with what's been said. Most likely they're just a strong player.
I've had many times in similar situations. In some groups my winrate has been well over 50%. That'd be using my own decks, which admittedly have a very high budget, but I also try to temper them significantly by avoiding lots of different things that I don't think make for fun games. Sometimes I'll play precons to avoid winning via budget, but I still usually have a pretty solid winrate with those as well. Being a strong player gives you lots of little advantages throughout the whole game.
I see three options though:
1) Git gud. Personally my own path to being gud was paved through playing the game for almost 20 years (commander for 11 of those). I think the best thing you can do is try to analyze your games, and maybe talk to the other players about what they think you should have done (especially the good players). But more than anything else, just play tons and tons of magic. Might also be worth watching people play the format on twitch or youtube and trying to find good plays.
2) Git upset. Probably not a great option ofc.
3) Giv up. You don't need to be good at commander, it's just a game, and one that takes a lot of effort to git gud at. It's totally reasonable to just accept that your winrate isn't going to be very high, and that's ok.