This is simple. You killed the battery by sitting with things on and the car not running. The alternator needs to turn to recharge the battery. The alternator can only turn if the engine is running. Also your A/C wasn't doing anything. You simply had the fan on. The only way your A/C is going to work is, again, if the engine is running.
The reason the car started for the tech was that it is relatively new, and once you got a jump the alternator recharged the battery.
Sitting with the radio and fan on, and the car not running is going to kill any battery. This isn't some crazy Subaru only thing.