There are actually two additional ways to win the scenario. Since I'm sure that you don't want me to spoil the suspense

, I will just provide hints. Both are tech related. 1) There is a Unit Killed event that provides the winning tech. 2) This is a non-stealable tech, but if you are the first Civ to discover this tech you will win the game.
I found out about the Unit killed event when I was successful in combat against one. Bribery doesn't trigger the event, I tried, it has to be either an offensive or defensive kill. Just as a warning, if you figure out the required unit, it won't be an easy kill because the AI will do everything possible to avoid combat, including disbanding, if the unit is severly injured.
I found out about the tech discovery aspect when another civ beat me to it.
Ken