It would be a valid implementation of C. But if you want to depend on the new behavior, which was previously undefined, then you would have to reference this ne

It would be a valid implementation of C. But if you want to depend on the new behavior, which was previously undefined, then you would have to reference this new specification. That effectively makes it a new language, because you would no longer reference the C specification, but this new one. (And if you don't want to depend on the new behavior, which was previously undefined, then there's not much point in using this particular implementation.)