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.
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.
As someone with both ADHD and autism, I want to touch on a perspective I think might be missing here (if others have commented similarly, excuse me for being a parrot). Both definitionally and experientially, autism and ADHD are both disabilities. Both come with strengths; autism makes me really good at learning patterns which helps with language learning, for example, and ADHD helps me have a large amount of focus however controlled on a specific topic. Yet, both are still disabling.
Autistic people constantly are confused by allistic (read: not autistic) people in their communication style as well as the overreliance on things like body language to convey meaning. Autistic people also deal with sensory overload issues.
ADHD people struggle to focus on tasks no matter how much they want to focus on them.
I could include more examples, but that's not the purpose of this comment. This could also be stemming from my autistic perspective where when I, specifically, use the word "neurodivergent," I use it to mean a collection of disabilities such as autism, ADHD, and OCD, and that's it. I simply view it as a label much like what the label of "queer" is today despite its troubled history.