1. Even the chart says to wait until the fourth quarter, and while technically we were in the fourth quarter, it was only by a matter of seconds. There was still enough time that it was quite possible the score would change again, which is why the chart says to wait until the fourth.
2. The chart is not really perfect. Say you are down 12, as ND was. The chart says go for it, because in order to tie the game (assuming the other team doesn't score), you need a TD+1, TD+2 and a FG. Well, if you need the TD+2 regardless, how does it benefit you to do that first? Since the score could always change, it is better to take the TD+1 (the sure thing) first, then when you get to the second TD you can go for 2 if the situation is the same or, as happened tonight, take the PAT again if the other team scores in the interim. Tonight is the textbook example for why the chart is wrong in this situation. (tl;dr: if you need to two TDs and one 2 conversion, wait for the second touchdown to minimize the chance that you needlessly leave a point on the field that you might want later on.)