I'm not really convinced. Sounds like a weak generic "modern life is to blame for everything" argument.
By everything I've heard from my parents and grandparents, I'd say teachers used to be stricter. My gut feeling is that older generations had to sit more still and be more focused in school. There's a lot of teaching through games and play and all sorts of interactive ways, which are more chilled than how it used to be in the past.
Wouldn't there be documented increased numbers of people with ADHD-like traits and behaviours in the generations that grew up during wars, if reduction in outdoor activity was a problem?
I'm not sure about mobile phones either. There are many people seeking ADHD diagnosis who've had problems their whole life but got their first mobile phone when they were adults. I got my first mobile phone when I was a teenager but I had my obsessions, i.e. activities I was really into, which kept me in the bedroom before the phones and remained obsessed despite having a phone (I don't have ADHD, just talking about my experiences and observations). I appreciate the mobile phone world is very different now as well as many other things but it's not that our childhoods were ideal before them. Trends and stupid ideas spread from person to person but they existed. Not everyone played super smart games in optimal amounts to facilitate their development and read books. I don't know... Somehow I don't find it convincing enough.