Setting aside MO's apparent misrepresentation of what was actually said...
The difference between a defender under the basket, and one away from the basket but with his back turned, is fundamental. Or definitional, to invent a word.
"Guarding is the act of legally placing the body in the path of an offensive opponent."
Under the basket, you're not in your opponent's path. He's going to the basket. You are not preventing him from going there.
Out front, however, the mere act of being between your opponent and where he wants to go disrupts his ability to get there. You are in his path. That you could defend more effectively if you were facing your opponent does not change that.