The word pressure is misused here. It's not positive pressure, but negative pressure. Vacuum effect would be a better description.
When the gas tank is full there is very little air inside. As the level of gasoline drops (the more you drive) the volume of gasoline is displaced by air in a normal situation.
If there is no air going inside the tank the air which was there initially gets "pulled apart". In other words, the same amount of air has to fill out a larger volume, therefore creating a vacuum effect inside the tank.
Unless something is pushing a lot of air inside the tank, there is no way you could end up with more air inside the tank filling in the volume of gasoline spent.
So yes, it is pressure, but not positive.
There is more (positive) pressure in the atmosphere which pushes air inside the tank when you open up the cap... trying to fill in the empty volume missing.
Put it this way:
Plastic water bottle.
Open the cap, turn it upside down. As the water exists the bottle, the walls of the bottle tend to cave in due to negative pressure created vs the atmospheric pressure (which is pushing the walls in).
The danger here is that the tank will collapse, not inflate and explode.