Is the memory-unsafe descriptor just saying it wasn't written in rust? Or does it specifically do some UB stuff which happens to work out so far?

Yes, "blazingly fast", but not in Rust! In fact, I have my own development model in C++, which allows me to write at approximately the same level of safety that Rust provides (at least, I think so, and asan/ubsan agree with me) - https://github.com/pg83/std/blob/master/README.md