I've not heard of anyone not identifying as a person, and I thought I was aware of a good range of identities and subcultures. At least in the UK, using "an autistic" as a noun would have you be heard as someone older and out of touch - perhaps well-meaning, perhaps not, but at least a bit insensitive.
Your report of the meaning of "autist" in some places I think rather strengthens my suggestion to avoid it.
I am otherkin and quite a few of my friends are also otherkin (or alterhuman, very nearly the same thing). There is also therianthropy under the otherkin umbrella, although I've seen that ordering get mixed up a lot. Most of the discourse about it I've seen is from the young/inexperienced.
> Your report of the meaning of "autist" in some places I think rather strengthens my suggestion to avoid it.
Maybe my definition of “person” isn’t quite standard, but I did have otherkin in mind; to me, a person doesn’t have to be human to be a person. My cat is a feline person, or near enough, Commander Data is a (fictional!) person, etc. Practically zero people want to give up their personhood, at least in everyday life, as it’s extremely disadvantageous and unpleasant. It’s in this sense that saying “an autistic person” rather than “an autistic” is much better: however you identify, you’re a person, an individual with rights and worthy of respect, and not a thing that one would only bother identifying with an adjective. A vital part of being a person is having more than one aspect. A lot of racial epithets are offensive for the same reason.
I do know at least one that explicitly doesn't identify as a person, because personhood does not come only with rights but with obligations, expectations, societal treatment and responsibility. I get what you mean (I haven't separated from my own concept of "person" either) but to me it's more inclusive not to require personhood for respect.
Your report of the meaning of "autist" in some places I think rather strengthens my suggestion to avoid it.