Yeah, but for me (and I think most people, although I suspect there's a lot of diversity from person to person) what comes "out of the brain" is noticeably diff

Yeah, but for me (and I think most people, although I suspect there's a lot of diversity from person to person) what comes "out of the brain" is noticeably different when writing by hand. Most of us can type a lot faster than we can write with a pencil. I doubt it is just the difference in speed (and I think Stephenson is simplifying here, not really saying that is the only difference), but that's part of it. A sentence written by hand has often been drafted, considered, and revised before it actually makes it to the paper. Whereas for somebody who can type, the first sentence that hits the page is often literally the first draft of it, as originally thought. (I still cannot imagine writing an entire novel this way myself, though...)