I think you missed the point above, the ref is going to blow the whistle for a few reasons and none of them have to do with possession or his physical body. As both are laying there for 2 full seconds without moving or making progress, the ref will blow the whistle to stop play for these reasons. 1) The player with said ball has given up futher progress 2) the saftey of the player, not good to have a guy hit while prone on another player after they have been laying there for a bit not trying to making forward progress, in this scenario forward progress would be establishing another foot or body part down. As a result of the whistle the officials would THEN determine whether the defender had the ball inbounds. Which in this unlikely scenario would be "yes" because it would be ruled a tackle by the receiver and play was whistled dead with both players in bound.