Going through that thread (a few posts back) it seems that “Rust is safe” (as seen in this submission title) was stated first by Torvalds. It wasn’t mentioned first by a “Rust aficianado”. So you would really have to ask Torvalds what he meant. But his mentioning of it (and this submission) obviously alludes to “safe” claims by the Rust project. Which has always been memory safety.

absence of UB is literally memory safety.

Rust proponents mean exactly "memory safety" when they say rust is safe because that is the only safety rust guarantees.