There could be many things working at once. Constraints often yield surprising benefits through innovation or forcing things to be distilled to the core important bits. In Stephenson's case, if he happens to record core info and not necessarily the full sentence he will write, which he can then finagle the right structure and prose for later, maybe that allows for better flow.

If that's indeed what he's doing, it might be like writing pseudo-code and then going back and translating it into the programming language medium of choice. The benefit of writing in a journal is that it's probably easier to do on a sunny day at the beach in the sand than it would be on a laptop or tablet. Also maybe seeing old ideas crossed out on paper helps later when looking back over it to get you into the same headspace.

Sometimes we over-optimize for aspect of a task (such as getting ideas down quick) in a way that's detrimental to the process itself when we step back and look at the whole.

what is the knuth airgap?