My understanding is that a problem was first identified at petrol stations when CB radio became popular. It was nothing to do with fires, the CB messed with the fuel metering in the pump leading to people being over or under charged. I believe the petrol companies started the fire risk story to stop people deliberately trying to mess up the pump while they were upgraded ti improve immunity. The ban on CB remained "just in case" and was later extended to phones.
If there is a risk to aircraft I supect it's more likely to be due to the potential for interference with the fuel metering but I don't know how that's done on a bowser.