A quirk of the English language means that the former is often interpreted as though it was the latter (IOW, the scope of a negation in English isn’t always everything after it in the sentence and nothing in front of it—e.g. “All that glisters is not gold”. To unambiguously express the former meaning you have to say “No X is Y.”
(Hadn’t seen this had already been discussed—never mind.)
(Hadn’t seen this had already been discussed—never mind.)