I suspect all languages without exception use some standard library functionality in at least a few of those sample programs, and most "standard" libraries aren't constrained to be self-hosting - so all of em use some native code, probably written in C or C++. I suspect that's true of rust too.

FFI is a fact of life. I can imagine it would be perverting the intent of the game if you explicitly used FFI to delegate the actual core of the benchmark program to C as opposed to using "standard" building blocks, but the distinciton is necessarily vague.