The route they were training for was to evac government personnel during an emergency (terrorism, incoming attack, etc.). ADS-B is live location whereas transponder is delayed. In a real scenario, you wouldn’t want to be transmitting live location, since whatever the emergency is likely involves targeting of VIP government personnel. But in training, that would not effect your training, since the ADS-B is for others benefit, and doesn’t change your situational awareness or capability.

edit: To add and make clear, the route will be known for a training or real situation, but it will be delayed. So for training, turning off the ADS-B does not protect the route information and that is why there is no reason to fly with it off for training.