Full disclosure, I am diagnosed with ADHD and take stimulants daily, and initially felt pretty defensive when reading your comments here and starting to read that article. I have some problems with the article and generally find that author pretty insufferable, but the article concludes that the risks of medically-supervised stimulant use are low enough that the author personally finds stimulants worth prescribing to patients who benefit from them.
Even Section 1 which you specifically referenced, and which admittedly made me pretty annoyed until I read it a few times, and even though the author certainly seems to be trying to insinuate that ADHD isn't real, doesn't really seem to be actually backing up that point. His main point or objection seems to be that ADHD is diagnosed using arbitrary subjective criteria rather than objective measurements like other spectrum disorders (isn't blood pressure also a normally distributed trait, where we* "arbitrarily" draw a line and say people on the wrong side of that line have hypertension and should be prescribed beta blockers?).
I'll admit to feeling a little argumentative after reading some of the comments here but I do genuinely want to understand these points better, and I feel like I must be missing some fundamental context or point that the article is making.
*for various definitions of "we", since different countries and organizations define hypertension differently.