For RMS, allowing non free code to be derived from free code is unethical, it goes against all the ideal of his movement. Indeed, from your standpoint, the thing he is fighting against seems trivial, but there are deeper consequences.

For RMS, non-free code period is unethical, regardless of its derivation.

He seems to think that gcc could output a incomplete AST that could be used to implement all necessary emacs features without being usable for compiler backends. The other emacs maintainers seem to think that is fighting with one hand tied behind their back.