I’m sure there is disagreement among some doctors over this, but ADHD in the current medical literature is not defined by brain function, it is defined by sustained dysfunction in life situations.
The brain forms differently in ADHD individuals. Certain regions have slower or stunted growth. If you’d like to know more, it’s explained in the beginning of this lecture: https://m.youtube.com/watch?v=SCAGc-rkIfo