I believe that as they already knew about the fog issue they should have diverted to another airport in the first place thus reducing the delay and giving people the chance to arrange collection etc before even leaving GVA
If the weather at SEN had closed in before your flight left GVA then the flight would simply have been cancelled - it would not have been rescheduled to go to another airport. Flights don't take off unless they expect to be able to land at their scheduled destination.
Weather diversions are for flights which have taken off for their scheduled destination but were unable to land there because of an unexpected weather deterioration after departure.
So - be thankful that the fog rolled back in after you had taken off. If it had come back earlier (or been forecast to come back earlier) you would have been stuck at GVA overnight.