It's not necessarily either a lie from the beginning nor some old truth that no longer is true.
It could just be that, yeah, all the stuff in that conflict, like, happened, but it was always more complicated than the narrative that survived. Oni always had complicated emotions and empathy and friendships, even with humans, but this didn't mean that the conflict didn't happen.
They were two cultures that were at war until one subjugated the other and centuries of oppression had followed. Maybe there were other ways to resolve things, ways toward peaceful coexistence, but no one tried the so now the fact of the matter is that oni have been oppressed for about a millennium and they need to move forward from that.