It could be, but I'm rather distrustful of such high numbers backstopped by nothing but the same exact set of MZ twins they've been using for lab rats for decad

It could be, but I'm rather distrustful of such high numbers backstopped by nothing but the same exact set of MZ twins they've been using for lab rats for decades now in various studies. How many monozygotic twins can you find with a robust diagnosis of ADHD, and how much money can you afford to spend to thoroughly exclude every other possible cause (TBI, sleep breathing disorders, deficiency, thyroid, autoimmune, neuropathies)? Quite expensive, nobody gonna do that. Science is about finding evidence to support a hypothesis, not some quaint notion of "truth". All that's needed from such studies is to support the "biological cause" conclusion so that you can substantiate the need for a pharmacological as opposed to talk therapy intervention. Those studies certainly support that, but all they really say "ok this is biology". Given a disorder is usually defined by being outside the normal range, as defined by two sigma interval, 5% of people have ADHD by definition. How many of those are due to bad genes, and other causes is actually very hard to say, as there isn't that much profit to be made in discerning this any further.