plenty of people with an average IQ have stupid beliefs, you don't have to have the IQ of a 5 year old to be deceived by a magician's illusion, or buy into Christianity or Creationism, you just have to be sufficiently ignorant about certain things... that's what I take 'stupid' to be referring to here (ruled by emotion, ignorant, credulous, etc.), not a literal developmental mental impairment of some kind.
in other words, I think most people have the capacity to not believe such stupid things as they do, but they haven't yet had that unlocked (such as by learning how to think properly). which means we'd [me and R.A Wilson) be calling stupid people who believe stupid things, not people who are 'retarded' or 'imbeciles' or some sort of technical definition of 'stupid' pertaining to their mental capacity.