It's worth noting that they were speaking about their personal experience in that paragraph. So probably for them, the "how do you get to that conclusion" is "trial and error".

But I've noticed that many many many people report the same effect, that there's something about pen-and-paper writing that's more effective for thought-lubrication. I resisted for a long time, but now I too am a convert to this school of thought.

Similarly almost everyone notices the downside: it's easier to reorder, reorganize, cross-link, etc, those thoughts, in a text editor (to say nothing of more sophisticated software tools). Some people have systems for doing complicated things with paper that they say mitigates this downside, but I am not currently one of them.

I guess it's possible that your brain just doesn't have this pattern in it. (That is, the pattern of finding pen-and-paper more effective for getting the thoughts to flow.) I mean, for all I know, maybe the huge silent majority doesn't have this pattern.