It Really is one Syndrome

Autistics are very diverse. Many people talk about the range from 'low functioning' to 'high functioning' (actually it's far from linear, and can change over time in an individual). There are autistics who talk in sentences at 18 months, autistics who never learn to talk, autistics who learn to talk late and end up with very good speech, autistics who loose speech skills, and many other variations, and that's just looking at speech (and just superficially).
With all this variation, some people think that certain 'types' of autistics have nothing in common, and barely (or not at all) qualify as having the same condition. And indeed, they can seem very different. When I was volunteering with autistic kids, I found a lot of variation - each kid was autistic in a unique way.
But when I started volunteering with all kinds of developmentally disabled kids, I saw even more diversity. Autistics are all unique, but all have various things in common with each other that they don't have in common with other disabled people.
And when I read stuff by other autistics, even people very different from me, they'll often describe things that are also true for me. Sure, we're all unique, but we have more in common with each other than with most people.
Ettina

able disabled autism diversity unable disabled