Sunfish,
I agree totally that the captain is not in a position to judge the seriousness of the threat. The upshot of this that he must assume the worst case scenario and act in a manner that protects those he is responsible for. Getting the aircraft on the ground and then sitting there keeping everybody on board is not in keeping with that.
Getting everyone a safe distance away from the aircraft would be the safe thing to do.
In this incident this was not done.
I question as to why the commander did not take a course of action that would have rendered his crew and passengers safe in the event of the threat being real.
Perhaps his course of action might have been influenced by his interpretation of the the Flight Crew Memo issued by FR management that passengers are to be kept on board in the event of a diversion?