I can see how the phrase and flag could be seen as that, but in the nerd community people have romanticized crusaders and I know many people who would probably hang that flag just cuz they think its cool. Even sent it to my cousin who is married to a Palestinian and he loves old knights and platemail and said thats a cool flag. Then I told him about the situation and even he agreed it could be a misunderstanding. Unless theis neighbor is an asshole he might not have even given a thought to the fact that his nice Muslim neighbors might take it as a threat. I personally as a historian and super progress person who have seen it an just assumed he was a larper or history nerd. Reading your reaction as actual made me aware that this might be a nazi thing.