mostly because that would just be terrible. if they're whole point is trying to get Thrawn back, how would Ezra have gotten back? doesn't make any sense. mainly i just don't want it to be the case.
another theory i've seen/heard is that it's Cal Kestis but i'm also not buying that.
most likely it's Star Killer (or a clone of him) which i'm only going with because i don't want it to be the other two and the name Marrok is similar to the full name of Galen Marek.