Of course: you can call a C function that's unsound, and that unsoundness can make your surrounding Rust program unsound.

This doesn't change the surrounding point however, which is Rust's philosophy of safeness by construction: if a particular piece of code is safe, then no safe (i.e., invariant-preserving) use of it can be unsafe. That alone makes Rust's security properties strictly better than those of C++, where even the modern safer paradigms can violate underlying invariants.

Unnecessary use of unsafe blocks is stylistically discouraged in Rust, so there's a lot less code that has the potential to harbor a memory safety bug, which makes bugs both less likely to exist and easier to spot when doing a code audit before introducing a dependency on third-party code.