To the level of a clinical diagnosis, yeah it seems quite likely to me that most people can’t discern autistic spectrum behaviour in their peers. I bet most people couldn't even accurately say what those behaviours would be.

"On the spectrum" has more or less become code for "introverted, obsessive, socially inept, and a little scary."

That can certainly be a syndrome, but the official DSM definition of autism is not based on those criteria.

Clinical autism tends to be much harsher in its presentation.

Clinical autism and clinical ADHD are notoriously difficult to diagnose in adults. In some countries it's even illegal to prescribe stims unless there's a childhood ADHD diagnostic.

Adults have been socialised to mask the more problematic behaviours, and they can also be unaware that what they're doing is masking: they can believe that everyone struggles like that.