It is in the rule book, I just quoted it to you, and the question isn’t possession, it’s control. To have possession you have to have control, with feet inbounds, but the rule in question only discusses control, not possession. Control doesn’t involve the feet.
I say Tate had control FIRST per this 20 second close-up YouTube clip linked below. You'll notice his left hand grabs the ball before Jennings can get a hold of it. Nowhere does it say you need TWO hands to control the ball although I still maintain that Tate bought his right hand into the ball before he hit the ground. Now with control of the ball, Tate lands FIRST and satisfies the rest of the completed pass rule from your Rules page: "A forward pass is complete when a receiver clearly possesses the pass and touches the ground with both feet inbounds while in possession of the ball. If a receiver would have landed inbounds with both feet but is carried or pushed out of bounds while maintaining possession of the ball, pass is complete at the out-of-bounds spot."
http://www.youtube.com/watch?feature=player_embedded&v=WDnSdkefYAA
http://www.youtube.com/watch?feature=player_embedded&v=WDnSdkefYAA