OK, here is my guess......
The tank uses an old fassioned piston pump. This type of pump can pump air at great pressure to depths of over 10metres, so the pressure is not a factor. The particular modle used would have to have two identical sized cylinders driven by the same fly wheel. One piston would be attached to a tube leading to the top of the tank, with the pistons output releasing the air into the atmosphere. This creates a vacume, drawing up the water from the large tank, which is refilled as required to compensate. Then, when the water gets close to the top, the second cyliders output is connected to the airstone. Due to the nature of this type of pump, the amound of air beeing pumped into the tank is equil to that beeing pumped out. Also, the air being pumped in would be fresh, rather than re-cycled. Just use non-return valves, in case of power failure e.c.t, so the water stays in the top tank should something happen to the pump. These pumps can handle unbelivable pressure, and run reliably for years. Assuming the top tank is sealed, I see no reason as to why my theory woulden't work, unless I overlooked something??
Rabbut