It's not and we don't. We can however run some lean in rust with things like https://github.com/conao3/lean-rs
Or you can use verus instead which does automatic mapping from the rust code to the proof where possible.
Not sure why you're responding with information that would imply they modeled Rust in Lean or vice versa. As I explained, their target was the compiled ELF binary, not Rust source. This way, they avoid including the entire Rust compiler and its as-yet formally undefined semantics into the trusted computing base. They lose guarantees about any compiled Signal binary that is not bit-for-bit this one, though.