Quote from TurboJusticeNo, it's simple convergent evolution. There's only a finite amount of possible designs in the game that are actually printable - so every once in a while, they will print one of your printable designs. But I'm sure the road that led to that design was very different.
Besides, it's against Wizards contracts to look at any custom designed cards. I don't think any designers are willing to put their jobs on the line to wade through the seas of garbage designs to find the gems, when they could simply try to design gems themselves.
Starting with "no" doesn't make you right and others wrong. That being said, I agree with your first point. However, beyond convergent evolution, during my 13+ years of being involved in the amateur design community, I have seen so many parallel designs between amateurs and professionals that it defies the laws of probability to the point of being miraculous. Not just singular cards either (which I would consider inevitable "convergent evolution"), but mechanics, particular groupings of mechanics, entire block themes, etc.
It's a bit on the naive side to believe that the contract clause restricting employees to look at this and similar sites exists for any other reason than to legally hedge WotC and, by extension, Hasbro. To do so is to put too much faith in humanity when it comes to its relation to economy. We all know that "the love of money is the root of all kinds of evil."
There's nothing stopping MTG specific WotC employees from looking at this and similar sites during their off time. And since no one is allowed to look at them, how could they tell on each other even if they recognized an amateur design without revealing her or him self. Ultimately, there is no accountability in this matter and one must simply come to one's own conclusion about it.
I for one think that they do look at this type of site. After all, Hasbro wants results that produce profit. And if the current staff doesn't produce, Hasbro will get another staff. Therefore, I think it's reasonable to assume that they're checking us out on a regular basis (as opposed to wading through tons of garbage). I have no problem with this because I think it makes for a better product.
I have personally designed a multitude of cards, themes, mechanics, etc, that have seen print. And have at times had to debate heavily in their defense. So I find it extremely vindicating when WotC implements an idea of mine (either purposefully or coincidentally). Especially when it was a radical idea that was ill received from me and yet we'll received from WotC.