There are really two independent ideas here. Taking advantage of early enthusiasm as a motivator doesn't preclude planning. It just means that both the planning and the fleshing-out have to be done in the crucial first N hours. For me, doing planning in this time often increases my enthusiasm.
Exactly. I couldn't get anything written if I didn't do some amount of deliberate pre-planning. If the planning and outlining is done properly, an essay should basically write itself.
Steve Pavlina explains that the method he'd been taught in school-- a highly structured writing process of organizing what to say before it's written-- tends to produce dull writing, but starting from enthusiasm results in articles which are a pleasure to write and are apt to be more fun and memorable to read.
This looks like PJ Eby territory-- it's about the importance of pleasure as a motivator.