@louis I think its more to do witht he adoption of this use of the term is recent, so maybe you just havent heard it yet. But using neurodivergent to mean “autistic spectrum exclusively” has been the norm and fairly widespread for about 2 years now.
As of the last year or two you will find medical sources definition neurodiverse as “someone with autism, or more generally…” as in even the scientific literature seems to be giving it special preference to autism as of late.
A “neurodivergent” person refers to a person on the autism spectrum or, more generally, to someone whose brain processes information in a way that is not typical of most individuals.