After the flight attendant told everyone to come gather 'round and announce the flight as being under weather diversion, that flight is off the schedule. It has become a "deadhead". From the point of view of passengers, it has been cancelled.
They gave the passengers the choice of taking it anyway and seeing how far it would go.
Meanwhile on the infamous 9/11, the flight(s) that had to land at Gander Newfoundland were already en route and passengers on board had no choice. Here the airlines had the obligation to get the passengers to their destinations eventually, or to refund the fares and return them to their origins.