These trucks, and millions of others, have been running with vented gas caps (little hole in them) for 60 years. The idea of a valve and spring cap is new (no gas fumes in the air) idea.
Gas doesn't "self siphone", the tank was over filled.
Gasoline in underground tanks is 40ish degrees. As soon as it gets pumped into your gas tank it starts to warm up to the air temp,70,90,...
Liquid expands as it is heated. That's why you don't fill the gas tank all the way up into the filler neck. It needs room to expand.
With an unvented tank it can build a lot of hydraulic pressure in the fuel system.
