I think the point is that by the nature of ADHD telling people to "get off the internet" as a solution to the problem would be more akin to telling the insomniac "just stop staying awake all the time!"
Unlike "stop staying awake" which is not actionable.
A book might not be a good suggestion for everyone, and finding the right "off internet thinking environment" is not easy for some, but it's doable for most.
That's the problem: you think "stop staying awake" is not actionable, while "leave your phone at home" is actionable for a person with ADHD.
Again, this is not different than saying "just be happy" to a depressed person. To that person, in that moment, "stop wasting time on the internet" is not actionable just as much, otherwise it would not be a problem in the first place.
I'm not an insomniac; nor do I have ADHD, as far I know.
I assure you that I cannot action an instruction to 'stop staying awake'. I certainly can 'leave [my] phone at home', and frequently do.