From the NFL rulebook Article 3, 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. If the ball is muffed after simultaneous touching by two such players, all the players of the passing team become eligible to catch the loose ball.
The only way Seattle could have won that game is if Golden Tate caught or simultaneously caught the football. Putting aside Tate's blatant offensive pass interference seconds before—or the bogus pass interference call to get Seattle in the red zone, or the phantom roughing the passer call that negated a Green Bay interception before that—M.D. Jennings pretty clearly caught the ball and Tate tried to wedge his arms in there—attempting to subsequently gain joint control, you might say. So that was clearly not a Seattle reception, simultaneous or otherwise.
Unfortunately for Green Bay, one official called it a touchback and another called it a touchdown. It was eventually ruled a touchdown and would be reviewed but there was no way it could be overruled. Seattle was given a touchdown and hopefully, finally, the League watched in horror as we have been all season.