Smart people often make dumb choices.
If by "intelligent" we mean the conventional thing -- learns new things easily, capable of reasoning through complex tasks, would do well in med school/law school/phd programs/finance/engineering, picks up creative disciplines quickly, etc -- then are we sure it's not exactly the other way around?
People become invested in cults and conspiracies for emotional reasons, not rational reasons, and conventionally intelligent people are extraordinarily good at post hoc rationalization. More importantly, they're often better at mitigating or managing some of the downsides (eg, maintaining good-enough status in a cult, avoiding talking about the conspiracy in certain circumstances, etc.).
At least, this has been my experience with some extended family who fell into a cult: the ones I could consider "smartest" were stuck in the cult the longest, because they could rationalize their way into an answer for everything.
At least in my experience.
Most other people make claims or may appear to be but in practice do not demonstrate it on a broad basis, as in your example.
Maybe a different terminology is needed to describe the latter case, 'selectively smart'?
From the Wikipedia:
> No True Scotsman, or appeal to purity, is an informal fallacy in which one attempts to protect their generalized statement from a falsifying counterexample by excluding the counterexample improperly.
So I think it's a No True Scotsman argument to make to say "well, if they joined a cult, they must not have been intelligent in the first place."
But now I suppose we're arguing over the definition of the No True Scotsman fallacy and that's just a bit too meta for me on a rainy Sunday afternoon, so I'm going to go walk my dog. :-)
It's not some abstract speculation. Even HN comments often demonstrate it one way or the other.