Sure, but I guess I don't really understand the argument. Why would Rust need to be involved earlier in the process? Isn't the point to have a way to compile rustc completely from source?

I guess it's cool to have multiple ways to get there, but that project would take way less work and get to the same end goal. It sounds to me like the author is trying to justify a cool project instead of trying to solve a real problem. That's completely fine, but I think most people would be happy with the mrustc project.