The context is "should I go for the 2-pt conversion?" When you're making that decision, you've already received 6 points for scoring the corresponding touchdown. The scenario presented is that you were down by 14, then you scored a touchdown to be down 8, and now have to decide whether to kick the PAT or go for 2 points. That's why it's correct to say "when you're down by 8". They're talking about a scenario where you'll still have to score another TD.