Oh sure, he might have made several, or many, drafts on scratch paper. But even then it is impressive. Many of these are around 12 pages of hand written text and math in ink with no corrections -- he famously used a Mont Blanc fountain pen. How many people could do that at all, even if they were copying from a rough draft? And there are so many -- more than 1300 EwDs!
Well he only needs to write one mistake-free page at a time. On page two, a mistake, you just stat page two over again.
But your point about doing 1300 of these is well-taken.
(For what it's worth, this would be easier with a fountain pen because a big selling point of them is they fly over the paper so easily compared to a ballpoint pen. I switched to a fountain pen, and I had to un-learn how hard to grip the pen and press on the page.)
Not to undersell things, but doing this for hundreds of pages is what everyone did before 1868, when the typewriter was invented. I think perhaps it's less about the physical act of doing it and more about the mental act of deciding to do it over and over again.
