I don't think it's productive to label anyone stupid, but a complete inability to properly weigh probability is damning to claims of intelligence. For instance, the scope of the conspiracy needed to fake the moon landings is completely implausible. And when you consider the incentive the USSR would have had in exposing it as a hoax, it goes from implausible to literally impossible. Similar arguments apply to the safety of vaccines, flat Earth, and other absurd hoaxes.

This is considerably different to the repetition of "common knowledge" which ultimately turns out to be some kind of urban legend.