It seems to me that this event attempts to show how de la Vallette's leadership lead to the Knights excellent defense of their fortress during this campaign. Since at this point in the game, if the Knights still exist, they may own Malta but not Rhodes, Rhodes but not Malta, or both of them (alone or in combination with other provinces) I don't think the event should check for province ownership. I also think the intent is that the improved fortress should be gained in a random province--if the Knights had happened to own Malta and Crete as in the original post, Crete might have been the focus of the campaign instead of Malta itself, so the -1 for the province should be put back. That will also help if the Knights still own Rhodes but not Malta--if the event specifies Malta, it's useless in such a case.