Also "If .. are implemented soundly" sounds harmless but simply means there is no safety guarantee (in contrast to Fil-C or formally verified C, for example). It relies on best-effort manual review. (but even without "unsafe" use anywhere, there are various issues in Rust's type system which would still allow UB but I agree that this is not that critical)

In C UB is part of the ISO language specification, but not necessarily part of a specific implementation of ISO C. If you argue that the ISO spec matters so much, I like to point out that Rust does not even have one, so from this perspective it is completely UB.