It's basically what DarkGodEM said, but in a broader sense.
Basically, they're good at managing their procrastination. It's honestly pretty easy to write one 2000 characters chapter/day if you can defeat your procrastination. It probably won't take much longer than 2h.
Sure, you'll need to edit that at least once, but the writing itself shouldn't take that long.
So like... If you write one day, and edit on the next one, and do this 6x/week, you can get 3 chapters done per week easily.
... For as long as you defeat the procrastination that is.
And of course, if you do struggle with procrastination, lack of attention, sometimes super attention and whatever, there are real chances of ADHD, but it's not like you have to have ADHD to struggle with procrastination... I mean, I do have it and I do struggle, but I also take medication for it anyways, so... Well, it's not necessarily a medical problem. Maybe it's just a commitment problem.