The key words are “secure” and “control”.
I just rewatched the catch a few times from the viewpoint of the camera in the end zone, stopping the action at the moment of the catch. Singleton “secures” the ball between his forearms as it comes down. It appears that he allows it to slide from the middle of his forearms into his hands as he goes out of bounds, thus the ball movement we see. However, at no time does it appear to me to be a loss of control, such as a bobble. It happens in one smooth movement. So from that perspective, I didn’t see anything warranting an overturn of the call on the field. Caveat: I am using my gold-tinted glasses.