I agree that there is such a thing as too early. As far as that being a major cause of ignorance toward autistics, I'm not so sure.
I think the reason for the push for early diagnoses is that early intervention is seen as the cure for autism. A lot of parents seem to regard ABA and such as "fixing the autism," and think of it like a cure. That's going to mean a lot of pressure for early diagnoses from many parents and groups like AS.
Unfortunately, when an early-diagnosed kid gets early interventions and turns out "normal," it could end up being attributed to the early intervention, rather than the kid maybe not being autistic in the first place, or being autistic but one who would've turned out relatively "normal" even without interventions. Kind of a self-reinforcing loop, unfortunately.