I can't be sure that runners weren't using a small external battery (which is a practical option for Rogaine type events).
A couple of other things to consider:
- there is a "battery save" mode in MapRunF that defaults to OFF. Testing would be required to see how much it extended battery life in a real event. (This tries to power down the GPS when the phone is stationary)
- MapRunG on a Garmin has not been tested in long events (not to my knowledge). I imagine that different watch models may have different battery capacity.