Thats not strictly true, when you open the gas tank it will equalise pressure in the buffer by boiling off liquid from the tank. You will still have a tank full of liquid/gas and one full of gas... unless the buffer is really cold then youll get liquid in both.

Either way youd have CO2 in both buffer and tank - not a full buffer and an empty tank.

Also as I remember opening the gas tank should only be done while arming the robot, which is done in the arena. If you are charging the buffer you are pretty much arming the robot. If the robot accidentally fired (maybe due to a leaky valve) in the pits there would be people shouting at you from all over the place.