I'm completely ignorant so forgive me if this is obvious: in the effort of the parent article - to compile rustc with gcc - will rustc still be feeding lots of code to LLVM, or would that code now be fed to gcc?
Similarly, you can build Clang using itself or using GCC. The resulting compiler should behave the same and produce the same machine code, even if its own machine code is somewhat different.
The produced binaries could still have artifacts from the original compiler in them, e.g. if "compiler built-in" libraries or standard libraries were compiled with the original compiler.
Both GCC and rustc use a multi-stage build process where the new compiler builds itself again, so you reach an idempotent state where no artifacts from the original compiler are left.