Try what? The only thing you try is score a touchdown and get an onsides kick late. That doesn't change if you miss the 2-pt later.
They scored with 3:00 left, and got the ball back with 1:49. It's not like there's enough time to really do anything different, either way.
Hey, now we're getting somewhere. You're agreeing in the first half of this that it doesn't matter when you try the 2-ponter: the chance of needing an onside is the same.
The issue is, why did the Cowboys have 1:40 left? It's because they scored the second TD really fast, and they did that because they knew they needed two scores.
Consider the possibilities after scoring the first TD:
1. Go for 2 and succeed. You're down 7. Your optimal strategy on the next drive is to drive down the field methodically, let the clock run between plays, don't take timeouts, and ideally tie the game with no time left on the clock.
2. Go for 2, fail. You're down 9. Your optimal strategy on the next drive is to score as fast as possible. Don't let the clock run between plays. You need to leave time on the clock in case you convert the onside kick.
3. Kick the xp. You're down 8. You do not know what your optimal strategy is. You don't know if you're going to need an onside kick or not. In this scenario, teams adopt strategy 1 above. Try to score at the end of the game, and hope you convert the 2-pointer. But if you fail, you haven't left yourself the onside kick as a fallback opportunity.