One reason why a lot of things are comorbid: generally, few mental things have "a single gene" but are manifestations of many gene differences and nature.
One reason why a lot of things are comorbid is just that more people are a little something than a lot something, and people who get diagnosed for one thing are more likely to have resources and reasons to be diagnosed for another something.
After rereading your msg, I'm concerned you're looking for some sort of thing like "If I'm sufficiently good at X I can't be Y"
I don't think these things work this way. Many folks on the spectrum share some social skills weaknesses, but social skills challenges are not really what makes them on the spectrum.
In all, all models wrong - some models useful. An attitude of viewing these things from the lens of asking if they help you understand yourself and navigate the world is probably best. I was really worried with labels before I kept repeating to myself 'all models wrong, some models useful'
Depression, ADHD, autism, all models. All 'wrong'.Still, useful sometimes. They aren't meant to be the be all end all though, and they only sometimes (maybe often) accurately represent reality, which is a bit of a silly statement, since there isn't one reality to it all.
There we go, I got more confusing sorry :)
Besides that, I think, generally, traits will be "a combination of reasons."
For example, even before stimulants folks w/ADHD had bursts of energy and attitude that can be similar to stimulant-driven quirks. Those quirks are probably just more common w/meds
My understanding of the term 'spectrum' in this context is less "a scale from 0-100%" and more "a collection of traits which, if you have enough of them, would suggest you fit into this larger category".
In terms of being being able to navigate social situations, look up 'masking'. Many people on the spectrum learn to behave (sometimes very effectively) in 'normal' ways in social situations but it's a set of learned scripts and behaviours rather than being instinctive.
Insofar as a a spectrum of impairment is referring to a spectrum of diificulty in socializing and a deficiency of adeptness at neurotypical theory of mind