Intelligence can be great if properly applied. We rightly celebrate (living and dead) smart people who utilized their cerebral talents for worthy purposes.
What we tend to forget sometimes, however, is that intelligence can just as easily be weaponized to seek decidedly unintelligent ends. More regrettable yet, such unworthy application is equally impressive.
It's impressive when a physicist makes an awe-inspiring cosmic breakthrough. You know what else is impressive? When a flat-earther contorts and bends and rationalizes a bogus theory into making sense.
Crazy as it sounds, I'd argue that flat-earthers are not necessarily unintelligent merely because they believe unintelligible things.
Indeed, possessing the mental capacity to twist the unintelligible into the intelligible conveys no small measure of intellectual dexterity.
And so it goes for all things. Rationalization, unfortunately, ensnares the smart and stupid alike. And a modern world where all information is never more than a click away only makes it a more accessible mental tool.
If you're ever wondered how some of the brainiest people you know can still believe such irrational things, there's your reason.
Flexing intellectual muscle unto itself doesn't do anyone much good unless they're also willing to embrace the eternal need for self-reflection and ever consider the possibility that they might be incorrect.