The lore is Sylvanas allowed for peace during the Legions invasion and shortly after in preparation for their counter attack. Saurfang agreed, with Genn’s actions at Stormheim in his mind.
The Horde was mobilizing and planning for the war Genn started while maintaining peace to replenish their forces and trick the Alliance. But Genn’s actions at Stormheim started the war by giving Saurfang the evidence he needed about the Alliance’s rampant warmongering. The fact that Sylvanas and Saurfang agreed to wait a moment before their counter attack does not negate who started it and when, which was Genn at Stormheim.