All the arguments put forward here are more or less correct.
The main consideration to be allowed for when open line refuelling ie overwing is that the fuel vapour that exists within the tank being refuelled will endeavour to exit to atmosphere from the open overwing fuelling cap rather than via the normal vent system.
It is for this reason that you cannot fuel so quickly; as the fuel entering the tank will tend to be blown back out of the filler cap orifice by the exiting fuel vapour.