Should be obvious, but there really isn’t any truth anymore. The receiver plants his right foot to start his comeback to the ball and the defensive player grabs him and pushes him back, preventing him from doing so. Uncatchable? Absolutely. But only because the receiver was interfered with.
Uncatchable cuz ball was gonna be intercepted even if gronk could go back but that is debateable re: the pushing and holding. Either way: revenge of the tuck rule