I suspect it matters very much what is on that screen. Music videos and action movies are practically designed for people with ADHD. Many video games are the same way.
On the other hand, there are some high quality documentaries and prerecorded college lecturers will not hav ethe issues you describe and as an avid (though very bad) GO player I personally feel that there is very little difference between playing on a screen and playing in front of a board.
Let's stop throwing around the word ADHD. It encourages people who just don't have the discipline to focus to give themselves an excuse, while lessening the perception of the problems for people who are actually debilitated by it.
There's no media-driven conspiracy to alter your mind. There has been an evolutionary trend to favor the animal who could quickly notice the approaching bear versus the animal too intent on its task. Conversely, human evolution has needed intense concentration to drive innovation in things like tool development, construction, and art.
There's media that caters to both. WIth action-packed movies and games thriving on quick attention-shifting, and grinding-based MMO's relying on the ability of players to do repetitive tasks for hours on end.
One could give the same attention-reducing arguments against books and for oral stories. That said, you shouldn't just make a blanket statement against all screen media. Teach your kids how to discriminate between what's good for them and what's not.
"There's no media-driven conspiracy to alter your mind."
I think that's wrong. Forget whether it's a conspiracy or not; many things are effective conspiracies. These days, media is designed to be sticky and most media is driven by advertisements, the goal of which are to make you think you're not good enough unless you buy certian things.
Video is a think pipe directly into your brain, and I believe there is evidence to suggest that "screen time" does alter the way your mind functions.
On the other hand, there are some high quality documentaries and prerecorded college lecturers will not hav ethe issues you describe and as an avid (though very bad) GO player I personally feel that there is very little difference between playing on a screen and playing in front of a board.