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.

(a) That's not what the text says.

(b) What compilers are doing does not match "ignore at execution", because they also use it to remove completely separate code.

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.

If that were what it does, I'd agree.

It isn't.

If you could give an example to actually illustrate what you mean, maybe I'd agree too, but it's a little tough when you don't...