I’ve read that the most fires at gas stations (related to refueling) happen when somebody enters the car while fueling. I don’t know if it’s true, but it has a point.

The potential static electricity from getting into the seat and the dilution of the gas in the air at that position is a more ignitable combination than holding a lit cigarette right at the nozzle. The draft from opening the door will make different dilutions, so it’s pretty sure that it is ignitable at some point around that position.

The risk of explosion is quite low at the pump. It’s actually worse if they step away to smoke, because depending on wind there will be a position somewhere in the vicinity of the pump where the gas is easier to ignite. Firing up a lighter there could be bad.

Anyway. There’s plenty of other good reasons not to smoke at a gas station.