I think it's the opposite, isn't it? Essays are "tentative"; they're a written document of the writer's grappling with their subject, which is why you get so many ideas while writing them.
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.
My best example: Woolf reads like stream of consciousness, but she was a meticulous planner: she obsessed over every word - her daily output was a mere 75-150 words.