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

I'd argue that while Rust has a high barrier to writing code due to lifetimes and other type constraints, its still quite easy to read.

(Kind of the inverse of perl)

While it's a lot easier to read then Perl, it's still not as easy as something like a Python.