My point is that I don't find it remotely feasible to write down unfiltered thoughts. They are necessarily dramatically filtered in terms of volume, because the speed of thought is magnitudes faster than the speed of writing. They are also filtered because putting things into words constitutes a transformation and distortion of the original thoughts, a crafting into words that are not quite the same as the original thoughts. Thirdly, the writing process introduces meta-thoughts about the writing, which when following "write down what pops into your head" very quickly leads to just writing about the writing.

Maybe other people are less conscious of the distortions and of their meta-thoughts. But for me it makes the exercise a rather artificial process, where it's difficult to produce anything authentic, anything close to the original thoughts.