IQ isn't an absolute score of intelligence, it's relative and assumes a normal distribution. Someone with a 200 IQ is not twice as intelligent as someone with a 100 IQ, they are ~7 standard deviations better on the score of a test designed to assess intelligence.

IQ is definitely not the end all-be-all of intelligence because you can train yourself to become better at taking IQ tests by taking IQ tests: that's what all those people with >180 IQ who are "the smartest people in the world" did. And of course intelligence is only part of the equation: people could be much smarter than Tesla but simply never become interested in electricity, and even very smart people can make mistakes and arrive at incorrect conclusions (perhaps due to ego as well - we are all human). There are also different forms of intelligence, as some people are much better than others at verbal/spatial/logical reasoning but perform averagely in other categories.

So in general I'd say IQ is not a good way to look at this. We should look more at the fact that mental illness can affect everyone, and if someone starts suddenly making provably false outlandish claims, that doesn't mean they weren't once a great scientist or very intelligent person.