I know how cxx works, and I am not misrepresenting anything.

cxx is currently unsound, and the author (dtolnay) has chimed in above with a fix they are considering that fixes it (https://news.ycombinator.com/item?id=24244121)

The API of the cxx crate is safe, and it can cause UB, therefore it is unsound.

It doesn't matter that the cxx crate API is a macro. Yes, this macro expands to unsafe code like you mention, but the problem is that this unsafe code is often "broken" (unsound).