Okay true.
Though back to the argument of Holocaust denial being made illegal, the only way you can do that is classifying it as hate speech.
Except that hate speech (unless it takes the form of advocacy of violence directed to "inciting or producing imminent lawless action and [which] is likely to incite or produce such action" https://en.wikipedia.org/wiki/Brandenburg_v._Ohio) is itself protected by the First Amendment. https://en.wikipedia.org/wiki/Matal_v._Tam
Admittedly, one can imagine a Supreme Court with a different jurisprudence in which hate speech--including Holocaust denial, which usually takes the form of claiming that "the Jews" or "the Zionists" have created a "myth" of the Holocaust--could be punished as "group libel." Indeed, Beauharnais v. Illinois https://supreme.justia.com/cases/federal/us/343/250/ has never been formally overruled. But it is pretty clear that it has long since been effectively overruled.