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.