It's not that you don't think you can stop the TD. It's that the only way you can "lose" is if they do score a TD and convert for 2. So you look forward that they already scored the TD and in that situation, with all the momentum, crowd going nuts, a tired D after that drive, is it easier to stop their 2, or would it be easier to convert your own where you just scored on them and you have momentum?
It's definitely a case where you have to imagine that scenario and the state your D would be in.