Since you are speaking about context, the chapter also explicitly says their demigod, Cenarius, had also been observing them and considered them the Legion's servants because of the presence of demon blood in their veins;
Cenarius had also been watching the newcomers, and he smelled the demon blood in their veins. Thinking that the green-skinned creatures were Legion servants, Cenarius attacked them alongside the night elves.
Furthermore, the text on the next page also says that;
Mannoroth’s mere presence in Ashenvale had already started affecting the orcs, particularly Grommash Hellscream and his Warsongs. Being near the pit lord had made them increasingly violent and aggressive. Their unbridled fury had drawn them into a war with Cenarius. The orcs stood no chance of winning, unless they had more power.
But as I said earlier, I am not here to influence your judgments. And I think I have spoiled enough information that any of you have enough room to work with when forming one.