I don't personally consider this a language issue as much as a people issue.
IIRC the current evaluation methodologies are heavily tied to the idea of a spectrum of traits, each with their own scale.
As you say, there are other spectrum diagnoses including autism, so "on the spectrum" is technically correct.
Which is why i consider the issue you seem to be describing as a person issue, not a language one.
A person using a descriptor or label with the intention of being an arsehole could just as easily use a different word or phrase.
Using something that isn't inherently considered offensive however, gives them some plausible deniability.
You can choose to find the phrase itself offensive and let people know of your opinion, but you should probably manage your expectations around how other people are using it so you can get an accurate reading on social intent.
That's phrased in such a way that it seems you think that the only reason to use "on the spectrum" is to purposely leave out the word autism.
If that's what you mean then i disagree, It's only my own anecdotal experience, but it's still at least one instance where what you are possibly suggesting is not true.
Coming from a "my interpretation is the only interpretation" viewpoint is an easy way to get confusing input from the world, at least in my personal experience.
I also disagree with this, offense is inherently subjective, I'd put good money on me not being the only person who thinks that.
I will however concede that "choose" was a bad choice of word on my part, as it's not always as simple as "choosing".
See my answer above about subjective opinion vs objective fact.
But it being offensive to you, regardless of intent, i can understand, which is what i was trying to address with :
I phrased that poorly, i think it would be better phrased as :
for example, if you know they don't intend to be offensive and you react with hostility, that's a valid choice, but it does come with consequences, knowing about the potential consequences beforehand means you can better prepare yourself.