I can understand that view; but by the same token, there cannot be any current flow without a return path of some sort back to the source. It matters not if the source is the PoCo or the generator.
No current flow = no shock hazard. As soon as there is current flow, the GFCI will trip- unless all of the current is returning via the neutral wire.
Naturally, no GFCI, on any system, will help you if you are the path between the hot and the neutral wires.
