Prolonged focus on a single activity isn't what people with ADHD have problems with. Rather, it's being able to control when they're focusing, and on what, and keeping that focus for any activity, not just the engaging ones.

Playing video games is thus usually a problem, not treatment - games are just more interesting in the moment than anything else you can or should be doing, so it's hard to stop playing. Ripping out everything that makes the game interesting and stimulating doesn't turn it into a therapeutic tool, it turns it into a bullshit chore that's done on a computer. In comparison, doing your taxes may seem more appealing, because while it's mind-dumbingly boring, it's at least not a make-believe time waster. Which I suppose could be of therapeutic value, if not for the fact that other videogames still exist.

(And so do books, TV shows, parties, substances, whatnot.)