Tyres are rubber, therefore don't conduct electricity. Neither does the ground/earth for that matter, or at least only very slightly.
Grounding or earthing in electrical terms, doesn't necessarily imply connecting to Mother Earth, but rather connecting to the chassis or frame of a radio or television or other electrical apparatus. Here's what Wikipedia says
"In electrical engineering, ground or earth is the reference point in an electrical circuit from which voltages are measured, a common return path for electric current, or a direct physical connection to the Earth."
Fuel flowing in pipes, especially certain types of plastic pipe or funnels, can build up a static electrical charge in that material, which when brought into very near proximity with the aircraft filler neck might allow the static charge to jump the gap i.e. cause a spark.