Intelligent people can easily believe things which seem “stupid” to people who believe something else.
I knew a very intelligent man who claimed if all the ice in Greenland melted, it could not possibly raise the level of the oceans as much as was claimed, because Greenland was just a small island.
I looked up the numbers, did the math, and convinced him he was wrong and the math showed the oceans could rise that much. He had never actually done the rather simple math. He just assumed “those people” were lying to him because they had lied to him about many other things.
I also explained one of the real lies was that the melting of the Greenland Ice sheet would happen quickly. That helped.
Some people are extremely intelligent about many things, and extremely ignorant about many others.
We are all ignorant, just about different things.
There are different degrees of ignorance. It takes a much greater level of ignorance to think that the Earth is flat than e.g. not knowing how semiconductors work. The "we're all equally smart/stupid, just in different ways" simply isn't true.