I've heard this statement made by JWs at meetings, as well. Where does this come from? I usually hear it from people who can barely read themselves. I suppose, in the absence of any other information to contradict them, Witnesses will sincerely believe that. Or, quite simply, because they believe they have 'the truth', all other knowledge is irrelevant. By default, they're the smartest; after all, why else are they the only ones who will survive the impending global holocaust? Propaganda technique: "Wise ones will take their stand on Jehovah's side."
This is one of the things that always bothered me even when I believed the doctrine--sweeping, broad statements like that. I always figured we were in such a tiny corner of the world. Where is our collectively brilliant intellect coming from? Awake! magazine? Holy spirit doesn't write those article about the ant, okay? JW information is cobbled together from various outside sources that the ordinary JW would probably be discouraged from reading too much. Given that new vocabulary is pretty rare in the literature, one's intelligence will not significantly improve by reading it. I noticed many an article that had wording that was clearly cut and pasted from a previous article, word for word. The publications often do this, paste a couple of paragraphs in from a previous article.
I've heard it before, but...you can't be the smartest if you're not even allowed to read or study whatever you want. That's just common sense. Even without referencing the repeated skewering of 'higher education', it is clear that that statement has no basis.