If you rewatch the play he is literally the only person lined up on the left side. All the other receivers are on the right side. Number 39 still decides not to cover him and just lets him run by until he panics and realizes Kelce is wide open. Dude was literally just guarding a patch of grass. I can't imagine watching Gronk come to my side of the line, with NO OTHER RECIEVER, and i dont react or see if anyone has him covered. Kelce got a free release. It was pitch and catch.