> What does it mean to not believe in IQ?
I believe that IQ exists, but I don't believe it's an effective measure of the things people typically expect from it once you get into average or above average scores.
Leaving aside the problem of the tests themselves and how they're conducted, I suspect that even a "perfect" test wouldn't predict how well a person would manage their life, perform in their job, or contribute positively to society.
At the low end, IQ testing can identify individuals with problems that will certainly cause them difficulties, but a person with a high IQ won't necessarily outperform someone with an average IQ in tasks that aren't IQ tests.