I don't know about anything Emacs internals but what a strange remark: "I recently wanted to change how `M-x' works. The command behind it is called `execute-extended-command'. It is written in C. This is disappointing for my desires to tinker, but not all together surprising either. It is a pretty central piece of the Emacs infrastructure."

It being a central piece of infrastructure I would expect it to be written in a higher level language for better maintainability.