The C standard allows for many different compiler implementations. Say you set in stone and publish to your users, a certain implementation specification.

You have now created a new spec. Really a new language. Your users will make programs that rely on this new specification. Such code will no longer be portable to other standard C-compilers. Their programs might only be well defined in your new language, not in standard C.

For instance he talks about a C that default initializes all memory to zero. A program that relies on that will no longer be portable to other C compilers. If its no longer portable, is it really the same language any more?