It seems beyond belief, at times. Like he was superhuman.
He didn't have to rely on publishers, of course. He had patrons. Dostoevsky famously wrote very fast because he HAD to - and you can tell reading his books (as much as you can, in translation, anyway) that he wrote them in a sort of white heat.
The polar opposite being Flaubert, who took that "mot just" thing to the utmost extreme.