Here's my take on it.
The pick up tube in the tank is located near the bottom of the tank so it is under gasoline all the time. The fuel delivery system is sealed. When the pump turns on it is going to start pushing whatever is in the line. That could be air in a new system or one that has been sitting a bit.
A vacuum is created in the line to the tant and gas begins to flow to the pump because of the sucking action.
Once the gas reaches the pump it is pushed ,just like the air that was before it.
It is better to have the pump located below the top of the tank and as close to the tank as possible as they push better then pull.