I think the lines between various types of brain things can be blurry....but there are distinct types of brain things (or distinct ways of classifying them, depending on context and purpose of classification), and based on our limited knowledge and currently simplistic classification of all brain things regardless of context (which I find very paradoxical), I agree with you that ASDs are not mental illnesses.
(That said, to highlight how I think about it outside of the simplistic classifications.....
There are people with schizophrenia who show lifelong symptoms of developmental disorder which have given rise to a theory that there is at least one type of schizophrenia that is a developmental disability involving psychosis...An alternative explanation for this is that there are people with schizophrenia who also have developmental disability as a separate brain thing that has been overlooked only because they have been diagnosed with schizophrenia. (But the alternative explanation calls attention to the fact that our definitions are somewhat arbitrary -- they lack contextual relevence in the name of being objective, but are informed heavily by unacknowledged contextual variables (like the affected individual's personal circumstances, what their life is like and what struggles they do or do not experience in the world; the individual perceptions and biases of clinicians and researchers, which are informed by cultural perceptions and bias; and also by the way that health and social support systems are organized) because we don't actually know what causes the symptoms that we attempt to group and separate into distinct disorders/conditions....How does one know the psychosis is unrelated to the developmental disability symtoms when it isn't really understood what's happening in the brain, or in the body more generally, to cause any of it?).
There is also a condition that is not officially recognized in diagnostic manuals (as far as I know) called "Multiple Complex Developmental Disorder" that is considered a childhood developmental disorder and includes thought disorder/psychosis as one of the core symptoms.....I suspect the group of children described as having MCDD has the same brain thing(s) as the adults with schizophrenia + a history of developmental disability. )
However, I only recently talked to someone who doesn't share this belief at all. Who things aspies are "crazy". It is these people who define it as a "disability".
I generally define ASDs as disabilities but it's not because I think that ASDers are broken or defective or "crazy" (or psychotic or experiencing thought disorder....such an interesting name "thought disorder" -- it never stops seeming odd to me that thoughts can be considered disordered, no matter how strange and detached from reality those thoughts may be).....there are a lot of reasons why, and I am not entirely certain about what all they are yet, let alone how to explain the ones I am certain of. But I will say:
I think of the term "disability" literally, as denoting a lack of a certain ability/abilitiesthat the majority of other people have ..... or of being unable to do some task or activity a particular way that most other people can do it (even if one can do the task/activity but only in a different way) in a world where if one can't do the task in that particular way that most people can do it, then one has no opportunity to do it at all.
ASDs can also be gifts, as they can confer abilities most other people don't have.
To me the disability and extraordinary/unusual ability parts of autism that I experience are neutral and factual and I think that disabilities and extraordinary/unusual abilities are both totally normal and common and acceptable parts of human diversity -- neither automatically good nor automatically bad. They just are.