This doesn't really sound sane to me. In this particular case, it would mean that if you throw away SBCL's compiler, SBCL's standard library, SBCL's extensions,

This doesn't really sound sane to me. In this particular case, it would mean that if you throw away SBCL's compiler, SBCL's standard library, SBCL's extensions, and replace them with something completely different, you still get the same Lisp implementation because all of the aforementioned elements aren't a part of what you call the implementation.