Quick and cheap explanation many use, but not the case. Severe autism is also on the rise, and that doesn't need special telescopes, it's just evident.

It's entirely possible that the ability of doctors to diagnose even severe autism has improved, for example a family doctor who sees a young child with very low IQ may not be surprised by undeveloped language skills and may not recognize that as autism specifically. It's also possible that the increasing destigmatizing of mental/developmental disorders and increased awareness of autism in general could result in more parents and doctors seeking proper assessments. The sad reality is that there are still places where autism is likely to be seen a spiritual condition, a witch's curse, or demonic possession.

What is the incidence per million over the last couple of decades?

>but not the case

like all things in life, it's not the only variable. but better recognition is absolutely a factor.