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.
Neurodivergent is an umbrella term that covers a whole bunch of diagnoses, not limited to just ADHD and Autism. This infographic sums it up pretty well in my opinion. Neurodivergent refers to anyone whose mind or functioning diverges from dominant societal norms, standards and expectations (neurotypical).