The logic wasn’t about Anduin being an agressor.
The logic was about Anduin not reining those that were indeed acting agressively.
If those in charge of the military are allowed to run rampant and freely doing whatever they please, then of what use is to be at peace with the one supposedly in charge?
Say for example that Genn or Rogers had been succesful in killing the leader of the Horde, alongside a sizeable chunk of the Horde military. Then what?
“Wait, but Anduin wanted peace!” is not an excuse.
What if the next time they attacked they were succesful? That suddenly Rogers bombarded Undercity in retaliation for Southshore.
Or Genn destroyed the Echo Isles in retaliation for Gilneas.
Why should the Horde be expected to keep their side of the deal, if when they suffer an attack such as the one in Stormheim, the ones responsible for it get away with it unpunished?
When Putress bombarded the Wrathgate, the Horde marched alongside the Alliance to kill him. When Krom’gar destroyed a druid location, Garrosh threw him of a mountain.
When Twilight Hammer started skinning NEs in Ashenvale, Varian DEMANDED they were prosecuted and turned over to Alliance justice unless Thrall wanted another war in their hands. And Thrall tried to comply (with the only objection being that the Horde would try to deal with them internally)
In fact, Varian DECLARED a war after the Wrathgate DESPITE the fact that the Horde had marched alongside him to punish the one that carried it out.
As far as the Horde was concerned, the attack that costed the Horde its fleet in Stormheim, and that tried to assasinate their leader, had the ones responsible for it getting away completely scott-free.
The fact that Anduin leadership had so many flaws and lackings, was one of the reasons as to why the Horde viewed conflict inevitable. And this peace but wet paper.
It certainly didn’t help that Anduin’s “decisiveness”, when it finally manifested, came in the form of “swarming” Orgrimmar with spies.