Not like real life chronology matters any, considering it's a fantasy universe with space ships that exist simultaneously alongside airships, which in turn exist alongside people riding horses and using catapults. The concept of war crimes could come to be at any point in the story, and it does. That's all that matters.
"This was a genocide and a war crime."
"Uhm it's a fantasy universe, so the moral compass and laws of people in Azeroth don't follow ours."
"Yeah, well the writers say those concepts do exist and Teldrassil was considered both."
"Uhhhh.. it's a pre-modern universe!"