Genn's already established hatred has been with him since cata. Yet he only attacked the Slyvanus after the perceived betrayal at the Broken Shore. I wonder why.... He might have never trusted the Horde but he trusted Varian's decision. That he had doubt about Slyvanus changes nothing.
Yes, they were planning to attack from the beginning because they had no reason to trust known betrayers(in their PoV) to be helpful to their goals. I never denied that. The quest makes it very clear Genn wanted to attack no matter what. That specific line i quoted was to prove that they were not told "Do not engage". I linked the whole quest for anyone to read it. I didn't cherrypick shit. It's linked right there for anyone to read it. You made the claim they were told "not to engage". I simply used to relevant quote to prove you wrong.
Genn was hunting Slyvanus(and the Horde) seeking revenge(for Varian, but also for Gilneas and his son) towards the end of the questline he eventually learns what her plans are. But prior to learning any of that, he was justified seeking retribution against a perceived betrayer that caused the death of his friend.