You think it's weird that someone speaking in their second language in a position of great international power is using language that you wouldn't use day to day?
There's no trick here. It's not a term he made up, it's one rooted in relevant international law. Stoltenberg said exactly what he meant: Russia's war is unjust. If you think that that's an odd position for the secretary general of NATO to express then I really don't know what to tell you