I'm a woman in my 40s who is probably autistic, but back then I was the wrong demographic and "too well behaved" to even consider diagnosis. I'm a typical example people think of when thinking about under diagnosis.

On the other hand, I work with people who have severe learning disabilities who also evaded diagnosis, or were diagnosed well into adulthood as diagnosis is difficult in someone so impaired. In another time, they would have been labelled with a grossly offensive term and just left. Better treatment of disabled people is probably another reason we see rising rates of diagnosis.

They're basically banking on people still thinking, autism is "intellectual disability, but quirkier and more difficult", while I have met "more severe" cases who did not had the ID part, they just were lucky to avoid the diagnosis for long, and thus people didn't pretend they're "too dumb to even learn to count to 10".