Doing a few things at a time is hardly an indictable offense. Self-compilation doesn't have to be anywhere near the start of the todo list. Relying on C++ infrastructure at compile time isn't a problem until you're trying to make all-purpose C++-free installs, and that's an entirely optional goal. The important part is having the runtime written in Rust.

Pointing out a language still needs C++ at compile time is a reasonable critique of "supplanting C++", but it's not a reasonable critique of "wanting to be a foundational language of the computing stack". Rust is the latter. (And even then it's too early to worry about compilers.) (And Rust is making good progress on compilers anyway.)