I find this type of science is infuriating. Monotropism as a theory of autism or adhd is equivalent to saying that “tendency to focus on a few things” is a theory of autism or adhd. You’re describing the symptoms, which explains almost nothing. A theory of autism would explain the underlying physical causes of the behavior.
That folks with Autism and ADHD may have large portions of their symptoms occur because they focus too much on some specific things, to the detriment of others - like emotional well being/regulation - can still be falsifiable (better than most psychiatric theories for sure!) and useful clinically.
Personally, it lines up with what I’ve seen and experienced.
That there is another (perhaps chemical, or brain structure) theory too doesn’t necessarily change that!