I love Stephenson, but I think he's deluding himself. The "buffer" is the brain. The point of writing is to get it out of the brain. Otherwise, to quote Forster: "How do I know what I think, until I see what I say."
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...)
If instead he'd said something like: "I just like the feel of pen to paper." Or "I've tried both pen/paper and a computer and there's something about using a computer that just doesn't work for me." that would make more sense. But this notion of a 'buffer' just doesn't.
Perhaps "deluding" was too pejorative; I'll grant that.
If you write a sentence per hour and a sentence per second, they will have different quality levels.
Either way, in the absence of very compelling counter evidence, I'll defer to the expert opinion of the author on how their brain works
Edit: the other salient concept which comes to mind is Marshall McLuhan's idea that "the medium is the message." McLuhan definitely believed that if you go and say something over broadcast media vs e.g. printed matter, what you say comes out very differently. Both the content and interpretation are influenced by the medium. He would probably agree there is a difference between typing and handwriting, too.
Edit2: there's at least one study which has found that handwriting engages more of the brain than typing does: https://www.ctvnews.ca/health/new-study-suggests-handwriting...
It isn't something you have to do, but then it's clearly not something you do.