The question "Why does something exist instead of nothing?" is different from "What caused the Universe?". The former question is not asking about causation in time.

Yes, that's true; I tried to distinguish them in my reply, because the point about time and causality doesn't really apply to the something vs. nothing question.

Suppose that your interlocutor grants that some event didn't need a cause, so that the Big Bang didn't violate causality. Well, the occurrence of no event also doesn't seem to need a cause. That is, causality would still have not been violated had nothing happened. So, it still seems reasonable to ask why something happened rather than nothing.

"Everything not forbidden is compulsory." – Murray Gell-Mann (from T.H. White)