Here it is!!
Foul on Last Play of Half or Game
1. On a foul by defense on last play of half or game, the down is replayed if penalty is accepted.
2. On a foul by the offense on last play of half or game, the down is not replayed and the play in which the foul is committed is nullified.
Exception: Fair catch interference, foul following change of possession, illegal touching. No score by offense counts.
This is why Pittsburgh couldn't decline the penalty, as it is on the offense on the last play of the game. If they rule there was an illegal forward pass (which was the call upon review), then the play doesn't count and the game is over.