The little hole inside the pressure vessel connects to a vent located around the central fill port on the valve and vents outside. This hole is very hard to see and is normally blocked by an internal seal so that the gas only vents out when you're filling the mag.
If you've ever had large BBQ style propane bottles filled this function is done by a small brass valve that the filler has to open manually. (spit 80% valve in this picture) TM fill valves have this functionality built in.
All TM mags have this, and as far as I know, only TM mags have this.