The problem with what you are saying is, if we accept that there can be any curbs on free speech at all, "how could anyone be stupid enough to believe that?" is not a useful guiding principle for setting the threshold at all.

The classic example of speech that is not covered by the First Amendment is crying "fire!" in a crowded theater. Of course it's rather crazy for everyone to trample each other trying to escape a fire that does not exist when they do not even smell smoke. That is completely beside the point. We govern the speech based on the damage it does, the intent to do damage, and the utter lack of any redeeming qualities to the speech.