Perfect pictures to prove what happened.
I think the physics of this is interesting. Allen definitely threw the ball BEHIND him, to a player who was BEHIND him.
HOWEVER-- Allen's forward momentum caused the ball to move forward relative to the (stationary) playing field. Yes, he released the ball at the 27, and the ball was caught at the 28.
As we can see by the pics, the RB remained well behind Allen, both before and after the throw. But the ball itself moved forward relative to the field, so it should have been a penalty.
In Australian Rules Football, this is legal. In the NFL, it's illegal.