That argument was absurd. Video games have such a high level of sensory output that it satisfies the ADD's need for a high level of sensory input.
I raised this with my son's teacher, and she gave the same stock reply you just did.
I then changed it to "plays chess" for over four hours without loss of concentration and maintaining an ability to sit quietly still, and the teacher's argument failed flat. Where's the sensory input in chess? ;)
You should have seen the look on her face. She clearly wasn't ready for THAT. She was dumbfounded, and her expression revealed it. (I still derive satisfaction from that memory.)
In most cases, it appears to be an internal discipline issue.
In fact, I always refer to ADD as "Adult Deficiency Disorder"--not enough adult supervision in the young kid's life.
Curiously, I, too, played a lot of chess. I have a particularly vicious case of ADD, to this very day. This might be something that ADD people do. We do a lot of mental gymnastics when playing chess. We're constantly analyzing patterns. Maybe that's the sensory input?
If you don't believe in ADD, ask my ex-wife. She'll tell ya. When I got on ritalin, as an adult, I became much easier to live with.