The pump is in a common chamber (the inside of the ballast section).
So it is pulling air from the inside of the ballast section which is connected to the anti syphon tube inside of the ballast tank.
The anti syphon tube pulls air from the top of the ballast section.
The pump can only pull enough air to fill the pressure tank which equals about 2/3 of the ballast tank in volume.
So when the ballast tank is at max pump pressure (to fill the pressure tank) the ballast tank is only 2/3 full.
If something were to happen the anti syphon tube would have to fill to the point where the air input was drawing water.
Even then the water from the anti syphon tube would enter the common air space in the ballast section.
It would have to fill to the point where the pump could suck the water.
So the odds are very good for the pump never seeing water.