Yes, but these are between-group differences. If you pool together the data from a bunch of people with ADHD and compare it to the data of a bunch of neurotypical people, you'll find consistent patterns. But this won't allow you to say anything meaningful about an individual person's scan—it can't be used for diagnostic purposes. The paper you linked describes efforts to use ML to fix this, but so far the results aren't all that impressive. It's not a matter of the FDA standing in the way. The technology isn't ready.