Nice diagnosis doctor assface.
Check the research.
Basically, ADHD isn't being able to sit still for a long time, it's not being able to focus on things that aren't fun.
For most brains, being able to get through say, homework, isn't a problem. You just sit down, figure, it's going to take about 20 minutes to do all the math problems and you go play video games.
For someone with ADHD, the brain is constantly craving rewards. So video games, movies, etc. all basically jam a fork into the pleasure centers of the brain. So ADHD kids can sit still and enjoy the fuck out of it. When the gears shift, and into say work mode, there's nothing jamming against the pleasure center and the mind loses focus.