Honestly, the ball broke the goal line and his forward progress had not been stopped. Did the whistle blow? Who knows? That would be the only thing that would make that play not a TD in my mind. If that were the case, then that was a mistake by the refs to blow the whistle that early.
I've said all of that to say this, I felt that Golson had stepped out of bounds before he fumbled. It was an extremely close call. The principle of the matter is this: if it is too close to clearly distinguish, do not overturn the call. That's what they did with the Golson fumble, and they followed suit with the Stanford goal line call.
Personally, if I were the replay official, I saw enough evidence to overturn the call. Obviously, the person in the booth did not. ND wins.