Depends how you look at it. They don't ignore it during translation but that's because they work hard to make the program "ignore it completely" (i.e., not spend a single cycle on the possibility) during execution.
If stripping code whose only relevance is in a UB situation doesn't amount to ignoring the UB situation completely, I don't know what does. It's literally the production of code that is completely ignorant of the UB situation.
