From his angle, I can def see how he could've mistaken it. The basket was between him and the ball, and if from his angle it was coming straight through and the net moved, I can see how a player can mistake it for a good shot. However, everyone else seemed to react like it was good too.
This reminds me of those good old times playing ball with a hoop with not net. It was always hard to know if the ball went in or not, which always led to some spirited discussions.