the defender.

It is a pretty easy concept, not sure why you are arguing semantics and trying to say it isn't.

If you are moving sideways and beat a man to the spot while you are moving the foul is on the other player. If the offensive player gets his head and shoulders in front of you and you move to that same spot it is a foul on you. Many commentators know this and will tell you exactly how it is. Van Gundy and Jackson obviously don't understand it.

P.S. If you really don't understand the difference between Mark Jackson saying it was a foul because he wasn't set and the rule saying you can be moving and still have legal guarding position then there is no point furthering this conversation. It is as plain as night and day and to argue otherwise is a desperate attempt to win an internet argument that you are completely wrong on. If that is what you really desire, congratulations you won the argument.