Memorizing phone numbers easily, doing arithmetic quickly, being fascinated by numbers, and being able to detect patterns in pictures rapidly, are traits that most people associate with high IQ, but that are highly associated with autism.

And, if you take an IQ test, you'll find it's full of questions testing how quickly you can memorize numbers, do arithmetic, and recognize patterns in pictures!

I think the idea that autism is correlated with intelligence is not the result of autism correlating with intelligence. It's the result of a cultural bias that doesn't understand what intelligence is, equates it with impressive "brute-force" autistic cognitive performance, and embeds that bias in our IQ tests. So if you test someone with autism spectrum, they score high, because the "IQ test" is partly an autism test!