"You do it in your own post, attributing a defined, binary, thing as "I am somewhere on the spectrum"

"You either have some quantity of illness or you don't."

I'm not sure what kind of argument you are making for (or against?) "binary" symptoms. The DSM-5 clearly lays out the spectrum. There is a conglomerate of effects caused by autism, and where you are on "the spectrum" is determined by how many of the symptoms you have, and their severity.

There is nothing wrong with someone claiming "I'm on the spectrum" if you don't know how or what they were diagnosed with. That language is consistent with the DSM. Unless they admitted to self-diagnosing, it seems wrong to assume someone is lying about their own experience.

"You can't just ascribe some quirky, possibly somewhat anti-social, behavior as being on the spectrum"

Quriky, somewhat anti-social behaviour (in your words) essentially is one of the dialogistic criteria. But nobody would be diagnosed with autism for that alone. Just like how autistic folks usually avoid eye contact. That doesn't mean they ALL avoid eye contact, and it also doesn't mean anyone who avoids eye contact is autistic. It's a wholistic diagnosis. One would need to be experiencing SEVERAL of the symptoms to receive an autism diagnosis. IME, the majority on the spectrum are indeed level 1, and high functioning, even to the point others might question if they are really autistic.

If you take issue with people self-diagnosing, I don't think anyone would disagree. But your combativeness in just discussing the topic kind of looks similar to people who refuse to accept that autism is really a thing ("there were no autisms back in my day" kind of thing).