I'm a bit wary about arguing with a psychiatrist, especially if it is Scott Alexander, but ADHD is clearly not just "low ability to concentrate". It is hard to explain to normal people without phrases like "executive function paralysis" but it is a severe illness, and not some variation of a norm.

I would also challenge his premise about boring monotonous work is ill suited for humans, hence the ones with ADHD are people who can't do it (as good as others). ADHD people can do boring monotonous work for hours, month after month, and often not even struggle with that. On the other hand some banal easy (and short!) tasks, which won't even register as a task for normal human, will leave ADHD person in shambles, unable to even think about it.

There is lots of bad shit going in on the ADHD brain, it is certainly not just "20% worse concentration" debuff.