I think that's only sometimes true. There are a number of well-written proscriptive essays where the author chooses to ignore paths that proved fruitless during her time drafting the essay. Furthermore, even personal essays can leave out sidebars that prove to be meaningless distractions. If I used Etherpad, I'm sure I'd write three times as many words as there are in the finished product; I remove more than I write, and I think that's the case for most people who take the craft seriously (regardless of skill, of which I have little beyond passion).

On the process of writing, however, you're spot on.