Something very cool about this GC is that it's written in F* (technically, a low-level subset called Low) and compiles to memory-safe C,* so it's kind of equivalent to using Rust but they don't have to give up C.

F/Low are also used in Project Everest, which ships formally verified crypto primitives in Firefox and other platforms.

What are those numbers on the sides for?

I think it is preprint, some latex templates put the numbers to make it easy for reviewers to refer them.