Referee said the punt was called a muffed punt and the rule is once it entered the end zone it was ruled a touch back. The reason people thought it was a safety was, #A. people thought he touched the ball in the field of play and live it did look like it touched his hand (replay was inconclusive) that is why Peppers #5 reacted and went after the football to recover it, he thought he touched it too. #B. The other reason people thought it was a safety is because Peppers made a football move with the football, after he picked the ball up in the end zone he started running sideways and then forward like he was trying to advance the football out of the end-zone. Because he made a football move many of us thought the rule was because he tried to advance the ball and was tackled in the end zone that it was a safety.
If you were there live you also saw that the side judge did recognize that Peppers went out of bounds because he threw his hat down on the sideline. Because they ruled he didn't touch it and that it entered the end zone, is why they ruled it a moot point, would not have mattered if he recovered the ball and ran it back for a touchdown the referees on the field interpreted the rule to be a muffed punt in to the end zone and the play was ruled a touch back and a dead ball. I think because they ruled it that he didn't touch it, and it was a dead ball, called a touch-back that there was no penalty because even though #5 Peppers stepped out of bounds and touched it, that it didn't matter once the ball was in the end zone.
The more egregious and realistic call missed on the field of play was the fact that the Michigan defense was off-sides on the final play of the game and clearly in the neutral zone on the freeze frame photo a lot of us have seen, on the replay they could have thrown the flag for off-sides on the review but chose not to. In my opinion if Minnesota was off-sides on that play on defense and the roles were reversed the officials would have thrown the flag after the fact.