The trick isn't just producing 1,000 words a day; it's
averaging 1,000
usable words per day. Once you take into account time spent in outlining, editing, and rewriting, that number starts to look a lot more impressive.
I myself average more than 2,000 words per day on days when I'm actually
writing, but that doesn't take into account the aforementioned outlining or editing time, nor does it take into account any dead time I might have between contracts or projects. I'm sure if you averaged my word count over the course of any given year, it would come to a lot less than that. And this is all I do; a lot of authors write
in addition to some form of day job.