I guess that's where I draw a line between "illness" and "disability", at least mentally (i.e. mental illness can get better, mental disability can't). I know some conditions have a very blurred boundary between "curable" and "incurable", depression being a notable example, but my gut feeling is that disabilities are permanent while illnesses either get better or worse (without treatment, I mean, for those that are kept in stasis by treatment).
I've not given this a huge amount of thought, though, so that game of semantics is probably full of holes.
I know what you mean, I had the same contradictory feelings (diagnosed two years ago aged 29) but the assessment label doesn't actually change anything about you, or much about life. If you have autism you've always had it. If you don't, you can still use strategies that help with autistic sensitivities/limitations, a lot of the techniques for mental limitations will help pretty much anyone.