With the last line I tried to condense (and oversimplify) about these ideas:

Not all writing is thinking: That all, even a lot, of writing, or parts of writing, is such that it will develop one's thinking much. For example most stuff I have to write, the dozen emails a day, the funding application boilerplates, the reports are stuff that don't really need (or deserve) much thinking but they have to get written. And even in the writing that deserves attention, there is stuff like grammar and spelling and surface style that usually take quite a bit of time after the ideas have been written down already.

Not all thinking is writing: for many cases writing is not a particularly efficient way to develop one's thinking, and e.g. visualizations, math, coding, discussions etc can be a lot better.