In my opinion I think it’s not as rare as people think it is, and TRUE asexuality often comes from autism. If you think about it, it makes sense. Sex is a very physically demanding and intimate activity that your average sperg would have trouble understanding. It makes sense that some spergs will have such trouble understanding to the point where they want nothing to do with it.
I have a friend who identifies as asexual, I used to think it was fake until I met her. She doesn’t like porn, masturbating, or sex. She’s not even disgusted by any of it morally, she just genuinely doesn’t enjoy any of it. And the more I observed her the more it seemed sorta similar to an autistic disliking a texture or a sound sensation.
There’s also the fake asexuals who just prefer porn to real sex.
I've often thought there are 6 explanations for asexuals: Trauma, addiction, medication, adolescence, shame, and I guess now "autism."
Trauma - People who are traumatized at a young age that, instead of becoming hypersexual like some victims do, they go the other way and become asexual, rejecting all the sex/sexuality related stuff.
Addiction - The porn addicts who are so far gone that nothing does it for them anymore except the most bizarre removed-from-reality fetishes you could possibly think of. They ID as asexual because their bodies are broken, but then they go into the "shame" one I want to mention in a moment.
Medication - People who are on medication that has a side effect that lowers their sex drive. When you're depressed or anxious and medication helps, usually it's a worthwhile tradeoff, but there's something to be said about "shit life syndrome" where you're only neurotic and miserable because the world you live in sucks, and medication isn't the actual answer. Especially when teenagers are medicated, that shit was NOT normal even a decade ago, and I think this leads a lot of them to identify with their medical side effects as part of their innate sense of self, either unconsciously because they just don't know it's a side effect, or consciously choosing to identify with it like a disability advocate.
Adolescence - Speaking of teenagers, I think a lot of younger people as they develop their sexuality (even before Current Era) have a lot of social pressure to know their sexuality and what they are and to "prove" it to their peers. Makes sense a lot of teenagers would go for being asexual, since it's the easiest to prove and is basically the equivalent of saying "idk leave me alone," except people would take it seriously and stop prying. There are adults like this too, who don't have a full understanding of their sexuality so they just call themselves asexual to buy time, but it's mostly teenagers.
Shame - Like I mentioned before in the porn addicts, some of them will ID as asexual because they're lying or ashamed of their addiction. Shame can kind of reach into all these other types too; shame over being assaulted and not wanting to confront it so you ID as asexual, shame over being heavily medicated, shame over being inexperienced, and especially as an "adolescent" asexual grows up and has no sexual experiences because they're ugly or rude or whatever, there's a shame to that where they just go "well I'm asexual, I didn't want to have a gf anyway," but you know that's not true. I think the lying aspect of a good chunk of asexuals could be attributed to shame.
Autism - I just figured that some people are genuinely uninterested in sex, like Nikola Tesla. Their bodies work fine, nobody's pressuring them to pick a side, there's no porn issue or drugs, but they just don't care about sex. I can absolutely see this being a symptom of autism for the reasons you described, but sometimes I think maybe it's just someone's personality. They could be "10% autistic" or they could just be a guy who doesn't want to have sex, and if it isn't hurting anyone I don't think the distinction needs to be drawn. Like the overlabeling problem with sexuality causes all those bullshit MOGAI things, overlabeling every unusual person as being autistic kind of misses the mark. You don't need a label unless it literally disables or disorders your life, and being asexual is probably the least offensive thing that you could have as far as a mental disorder goes. If I had to pick between hallucinations, fits of rage, seizures, or not having sex, I'm picking "not having sex" every time.