“I think it’s mostly, if not all, a matter of better diagnostic tools and procedures.”

This is the winner right here. If you ask your parents or grandparents about people they went to school with who were “a little weird” or “really shy” or “really into trains” or whatever it is. It was all just undiagnosed autism. The percentage of people with autism hasn’t changed. The percentage of people correctly diagnosed has. And of course it’s a spectrum. In the past they only diagnosed severely autistic people. Now with more understanding of it, we realize that it comes in several mild flavors as well.