This ruling is just saying Chromium loses only abilities it had before its activated ability (that includes "loses all abilities") resolves. Abilities that he gains after aren't "lost" because of this effect.
So, if you enchant Chromium with Prodigious Growth after activating its ability, Chromium will still get Trample even though he has lost all abilities before.
I'm confused on how that works and does it keep whatever the ability(s) are forever after? (Till end of game or killed ofcoarse)
So, if you enchant Chromium with Prodigious Growth after activating its ability, Chromium will still get Trample even though he has lost all abilities before.