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.

So it doesn't count if one guy has it and the other guy goes in and grabs it from him. It only counts if they both caught the ball together at the same time and neither one ever wrestled away possession.

That is not what happened in the Tate-Jennings play.

You can see it clearly in this screenshot (below) as they're falling to the ground. Jennings is the Packers player farthest to the left. The ball is in his chest. Tate's left arm is tangled with Jennings' arms, but Tate's right arm (the red circle) is clearly off the ball and just hanging there.

There's no way that can be a simultaneous catch. Tate only gains equal possession of the ball after they hit the ground, which, by rule, is not a simultaneous catch:

ESPN

So that's where the replacement refs were wrong. Either the refs didn't know the rule, or they didn't see that Tate didn't have possession until they hit the ground.

But wait, there's more. What about replay?

The problem is that you cannot review a simultaneous catch. Whatever the refs called on the field was going to be the final decision no matter what. On the field, one ref ruled a touchdown, another ref ruled a touchback, and we only figured out what the official call was when Seattle won the game. And that was that.

It's almost darkly comic — that an NFL game in the era of instant replay was decided on one of the rare plays that is unviewable, and the referee making the judgement was a replacement ref who usually officiates Division III games.