Tatum has been great on defense, but this team is more switch and scheme dependent than any defense I've ever seen in the pros. No one on the team is defending anyone straight up without help, and that's especially true with Durant.
Normally a guy like Durant would just move, and use picks to get switches. Every time he goes near another defender or a pick, he's getting beaten on and rushed by two guys. There is a reason Bruce Brown is scoring over 20 ppg in the series. The whole team is committed to not giving Durant anything.