Disclaimer: I have no quarrel with the mods using the term in the creation of this community. I understand why they chose it, as even if they share my disagreement with the term when applied to ADHD, there's not really a better inclusive term. "Mental illness" is really the only other option, and naming a community that would probably invite darker discussions that the mods might not be prepared to handle.
Another disclaimer: I think the term is perfectly valid when applied to autism, as autism is not, to the best of my knowledge, a mental illness so much as a difference in processing. Being autistic is only "bad" in the sense that our society discourages autistic traits. (Apologies if this is wrong; I'm neither autistic nor especially knowledgeable about autism.)
The term "neurodivergent" implies that there's nothing wrong with you if you have ADHD--you're just special and different. But my ADHD is an illness that requires treatment. A lot of people will tell you that the only reason ADHDers struggle is because society is set up wrong, but I don't think that's true, at least for me. Being unable to remember anything, unable to self-start, and hypersensitive to rejection would be massive problems in any world. Sure, the world today is particularly brutal for ADHDers in a way we could probably mitigate if we reorganized society to be kinder, but that doesn't mean ADHD isn't ultimately a disorder that some people need to treat with medication and therapy.
If you've read everything I have to say here and still don't believe ADHD could possibly be a mental illness, I'm not sure we can have a productive discussion, but I'm going to try anyway.
I would argue that any brain difference that makes you try to kill yourself is a mental illness. I have tried six different times to off myself as a direct result of rejection-sensitive dysphoria, which is an ADHD symptom. Perhaps not everyone's ADHD counts as a mental illness, but I believe mine does.
Besides, where is your proof that ADHD is a neurodivergence and not an illness? So far you've simply asserted this. Are there any studies? Is there any scientific consensus? To the best of my knowledge, neurodivergent is not a commonly accepted scientific term, so I don't see how that could be the case.
I think it might be different and present differently in everyone. I know so actually. So I might not classify my ADHD as a mental illness and rather a neurodivergence. You can view yours your way.