To me, exposition and backstory are two different things, but you may not see them the same way. Exposition is laying big structural story framework (could be past, present or future), while backstory is fill-in parenthetical stuff that is useful and fun, if not necessarily critical to the tale. It's hard to quantify the difference, but I know it when I write it.
The advice has a long pedigree: my first publisher, James Frenkel, passed it on to me when I was writing "The Throne of Scone"; I recently reminded him of it and he said HE'd gotten it from sci-fi author Vernor Vinge, who'd gotten it from someone he couldn't remember. So it's Ancient Tribal Wisdom for sure, being handed down in the ancient tribal style.
Anyway, it's the best piece of writing advice I ever got, and I thank whoever originated it, and I pass it along every chance I get.