It is pretty well understood that people with true ADD and ADHD have dopamine deficiency and issues with dopamine regulation and re-uptake in the brain. This has been highlighted in many studies. While I understand that there is an epidemic of over-diagnosis of ADHD, it does not make it any less of a legitimate medical condition in which the only treatment for it, is to increase the usable dopamine in the brain. One of the tell tail signs of ADD/HD is that the first time one with dopamine deficiency takes an amphetamine or one of it's derivatives, they realize that they are completely clear and it's like they come out of a fog in their brain, it's more profound than just the feeling of I can get stuff done. Again that alone is not effective as a single point to diagnose attention disorders but it is a strong indicator that there could be a dopamine deficiency.

TLDR it is a true disorder, it is over-diagnosed but that does not make it any less a chemical disorder of the brain.