As far as I remember, he doesn't really advertise being an otherworlder, so it's pointless to bring these things up. What is important is that saying a given people used to oppress yours in the distant past will be met with a shrug of the shoulders from an educated man like Amami. After all, human history is a litany of such things, to the point that if you name any 2 nations, you can most likely find a period in which one wronged the other and probably vice versa. It's not like he's helping the fantasy equivalent of Nazi Germany, so that won't impress him. He should have explained this to her.