Natural language does not have a specification, while both C and assembly do.
The C specification is orders of magnitude more complex and is much less defined than assembly. Arguably, the same could be said comparing natural language with C.
I admit that's mostly philosphical. But I think saying 'C can autogenerate reliable assembly, therefore a specification can autogenerate reliable C' is also about two different problems.
It's an interesting question and point, but those are two different things and there is no reason to think you'll get the same results. Why not compile from natural language, if that theory is true?