I stand corrected. It should have been an INT. I thought all the player needed to do was end up with his hand around the ball. That is not correct. Here is the rule:
"Item 5: Simultaneous Catch. If a pass is caught simultaneously by two eligible opponents, and both players
retain it, the ball belongs to the passers. It is not a simultaneous catch if a player gains control first and an
opponent subsequently gains joint control."
It's even more obvious watching the replays. Jennings had control and Tate only had one arm in there until they hit the ground. The first official in called a touchback signal.