Some schools (Penn comes to mind) have been more transparent about the defer to admit path.
It’s not terribly different from a school creating a WL of 850 students when the freshman class size is 500.
But some kids will get in after a deferral (or later, WL). Not most. The advice here is designed to makes your odds as good as they can be in a game where the odds are against you.
I DO think it’s too bad that most people don’t know this. I certainly didn’t! It was my quest for this (in the hours following a deferral!) that led me to CC years ago.