From what I understood in MCU time travel rules, they cannot travel back to before the snap to stop the snap because they cannot change what already happened. It had nothing to do with Tony's deal.
Basically when they travel back to the past, it now becomes their present and the future they came from becomes their past. My feeble understanding of this is:
- I travel to yesterday.
- I arrive to "yesterday". At this point, my point of origin (tomorrow) is now actually in my past.
- If I change things today, it does not affect my tomorrow because that is another tomorrow.
- I can travel back again to my tomorrow by actually moving backwards further.
Another way of explaining it is cause and effect:
Cause: I traveled back in time from Tomorrow
Effect: I am now in Yesterday
Effect cannot precede the Cause, so therefore, my "time from Tomorrow" is now in the past.
If that's how it is, I think it's pretty clever since it avoids any paradoxes from happening.