It is illegal for any gas system to vent into open air that is why you have charcoal canisters.
So a line used by another drain is a lie.
Most problems with back spray is from the type of pump used and the angle the pump is held.
Not all gas stations are the same and some will cause the pump to keep shutting off or spraying back by the angle of the pump nozzel.
Given that we're talking about car dealers, you certainly could be right.
I gather from the dealers discussion that the tank vents through the charcoal canister and into the drain line. There's no reason you couldn't design it that way, I guess. Houses use common drain-waste-vent pipes.
I used 4 gas stations of different brands and the problem occurred at all 4 regardless of the angle of the pump handle. Previously, I had had no problem with gassing up at these stations. Besides, the liquid does not come back out the filler pipe but rather a 1 cm wide "blow hole" above and to the left of the fuel filler hole. They said that this is an outlet of the vent system (presumably for tank vent air that's come through the cannister). I don't see how holding the pump handle the wrong way could cause gas to go in the filler pipe and back out the drain-vent pipe .