First thing when we get to silithus is attack goblins. This is the first act of agression between the factions after legion went down. I find it impossible that there wasn't any channels between the faction to discuss. Blaming SI7 would ask me to make the leap that SI7 attacks horde without the permission from Anduin or behind his back.
And again, breaking the cease fire instantly after legion goes down is just mind boggling. I don't think what happens in Teldrassil could be considered the start of a war, it might be the first big major loss to either side but thats what happens when two sides start one upping each other and this one started for azerite. Towards the end of silithus quests Khadgar gives up too saying: "I had hoped that after the war against the Legion, the people of Azeroth would build a new future together. But now the unity we forged has been shattered. As the life essence of Azeroth bleeds out, the leaders of the Alliance and Horde scrable to claim it as a resource. Blades are drawn, and the tides of war are rising once again." Khadgar giving up and saying this is the start of the war for me, not a big battle. It is the abandoning of the idea of an unified Azeroth.
Even if you disagree with all what i'm saying i don't think there's anything that would make me think that the pre-silithus cutscene and the quests after make sense. Talking about sacrifice and horrors of legion, then right after basically killing goblin grunt workers is just bad writing. They could have relayed the same info without having to kill npcs from opposite faction, they didn't which should matter.