I still suspect that yes, it was under-diagnosed or overlooked for a long time, but that ultimately it's a disorder that is, for most people who have it, a result of a fairly-large section of the ordinary human brain-development spectrum being badly mis-matched with modern society, in a way that it hadn't been in the (perhaps somewhat distant) past. Since differences in brain development aren't really considered disorders unless they cause a problem in ordinary life, if ordinary life itself changes to put different sorts of demands on people, then it seems plausible that could create disorders where there were none before. I'm pretty sure that's a lot of what's going on with AD(H)D.
I mean, if you look at early 20th century jobs most similar to office jobs today, they look from the outside, and are depicted by contemporary media, as crushingly dull—but, they had the benefit of also being highly repetitive, stack of papers to go through, do the thing you do, put it in the outbox, move to the next stack. Now everything's every bit as dull, but we're also expected to juggle a much broader set of not-terribly-similar tasks, often taking on little bits of what used to be the jobs of entire, dedicated humans before computerization "freed" companies to smear those tasks across their entire workforce. That seems like a recipe for failure for someone with ADHD-brain.
I mean, if you look at early 20th century jobs most similar to office jobs today, they look from the outside, and are depicted by contemporary media, as crushingly dull—but, they had the benefit of also being highly repetitive, stack of papers to go through, do the thing you do, put it in the outbox, move to the next stack. Now everything's every bit as dull, but we're also expected to juggle a much broader set of not-terribly-similar tasks, often taking on little bits of what used to be the jobs of entire, dedicated humans before computerization "freed" companies to smear those tasks across their entire workforce. That seems like a recipe for failure for someone with ADHD-brain.