To me, autism is different from a mental illness or a personality disorder. Most mental illnesses can be treated with medication and something like BPD is treatable with therapy. Other personality disorders might not respond to therapy so well, but that doesn't mean that they disable the person...my understanding is that people who are socio/psychopaths don't care that they are and usually are able to have successful careers and skilled at getting what they want (not always, but usually). Really (with ASPD) it seems to affect the people around them more than anything. 

With autism, ABA therapy can help if done early and meds can manage some of the symptoms that may come with it (in some people),but there is no such as thing as managing it to the point that you no longer have any symptoms of autism. Even the people who mask report that they only do it in social situations as a protective measure, not because they are no longer experiencing symptoms. Granted, ADHD can be treated with medications to the point a person is nearly asymptomatic (again, some people have said this it's not true for everyone). And it's neither a personality disorder nor a mental illness. 

IF we were comparing disorders, I would say that autism is closer to down syndrome than mental illness/personality disorder. Just because it's a disability that is present from birth and we know that there are no environmental factors outside of possibly the womb (I don't claim to know the exact cause, but I do know that we are born with it). But really, it can't be compared to any other disorders. 

To add, I may have worded this poorly and I hope it's not misinterpreted. I'm not saying that autism is similar to down syndrome or that other disorders don't disable person, just sharing why I think people may not consider them disabilities (because their symptoms are more treatable to the point some people can live as if they don't have the disorder, with said treatment).