The bootstrap seed, https://github.com/oriansj/bootstrap-seeds/blob/master/POSIX..., is a tiny interpreter that takes a much larger program written in a special-purpose, bytecode-based language. This proceeds in turn once or twice more--special purpose program generating another interpreter for another special-purpose language--until you end up with a minimal Scheme interpreter, which then can be used to execute a C compiler program.

All of this is incredible work, but a minimal C-subset compiler in under 512 bytes of x86 assembly seems like a unique achievement as it includes non-trivial parsing and linking phases not required for that hex0 interpreter.