Just the fact that you feel so offended about being called "mentally ill" shows how you feel about actually mentally ill people, like depressed people. That's all there is to it.
My objections are due to the societal biases that others hold against mentally ill people. My problem with calling Autism a mental illness is twofold. 1. It is factually and scientifically incorrect. Full stop. End of story. This should be enough for you, but whatever. 2. When you call Autism a mental illness you take a group of people who struggle and suffer and loop them in with another group of people that struggle suffer. Autism has enough social stigma attached to it already. When you apply more social stigma to it by attaching other labels to it, you are actively causing more harm than good.
It would be one thing to offend based on a factual argument, but you have neither facts nor a moral/ethical argument on your side. There is no reason for one to call Autism a mental illness other than a factual misunderstanding of Autism. To cling to that ignorance and claim superiority is both arrogant and egotistical. It lacks any semblance of empathy whatsoever.
Also, "disorder" is a more neutral word than "illness". Strictly speaking, it just means something is abnormal, not necessarily dysfunctional.