To answer this question we simply need to look where we have seen weak Alien X
Vilgax + Alien X
Atomix + Alien X
You know why are they both weaker than Alien X
Because they are fusion
So what if
Alien X in X-Tinction
Is a fusion of .....
Leave that point here a bit
Where have we seen fusions other than that of Ben 10k
Why Ben transformed into Stinkarms?
Because Omnitrix was damaged
Now returning to What if... point
What if after Vilgax damaged Ben's Omnitrix
Ben might have transformed into a fusion of Human And bit leaked DNA of Alien X
As this is not official if you are reading this tell me about your thoughts please please