If Mallory were to discover a blind spot in the semantic diff logic

This is a very big stretch IMO. That xz change wasn't actually the exploit, it was just used to make the exploit less detectable. And it was added by people with commit access so it didn't even have to go through code review.

On top of that, code review is not magic. It's easy to get bugs past it hiding in plain sight (if that wasn't the case Linux would be bug free!).

Can you think of an actually realistic example?