Adding air to a tank truck does not increase the pressure on the truck, unless you add compressed air. "agitating the fuel into the air" is a silly statement that has no meaning.
You do not seem to understand the fact that an environment of pure fuel will not ignite. There are 3 items necessary for a fire. Fuel, air and an ignition source. The mixture of the fuel and air have to be between the lower explosive level (LEL)and the upper explosive level(UEL) (a measure of the amount of amount of fuel in the air expressed as a percentage)
When a truck is full of gasoline, it has a 100% ratio, which is to high for combustion.
True, adding air to the tank truck does not increase the pressure on the truck (DUH< BONEHEAD< I NEVER SAID IT DID)but will allow a certain portion of the gasoline to evaporate into the air and become within the LEL and UEL, and become an explosive mixture. Agitation will allow the vapor to more readily mix in the air to acheive the level necessary for combustion. The containment of the tanker will allow a pressure increase once the gaseous mixture is ignited.
Enough educating the uneducated.