Wasn't the spectrum expanded in the last 20 years? That in itself would raise the numbers.
If I may play Devil's advocate, a level 1 or even a level 2 autistic person back in the day may have been seen as a bit eccentric or weird, but would overall pass for a neurotypical person and manage to hold a job, have a normal life if with some difficulty. So boomers probably knew quite a few people on the spectrum but just thought of them as just odd but good people.
I grew up in a place where the word autism isn't even in the vocabulary and thinking back, there's at least a few people I can now look back on and say they were autistic, even if we didn't know the term then.
Yeah, back then "autism" was a full-on disorder. It had to be so strong that it impacted your ability to participate in society. Now we realize that it's more a character quirk with individual strengths and weaknesses, that it's worthwhile to be aware of, even when you manage to navigate society.
Edit: Ouch, I'm apparently bad at being autistic, had no idea there were actual levels, thought it was a poor approximation of the spectrum by applying levels to something I thought of as more complex than that. My bad.
Actually I kind of have lol. I guess I don't have an expert on hand to tell me it's because of my being on the spectrum but I do require a lot more support the past few years. Possibly related to COVID?