Ok, as much as I am in the camp that believes Genn ironically did the Horde a solid in stoping Sylvanas, much to my annoyance, the fact of the matter is that Genn had no idea why Sylvanas was in Stormhiem. With the troop numbers and fleet she had she could have easily been moving forces against the Legion. Genn wasn’t aware what Sylvanas was up to till they tracked her to the Village outside Eyir’s temple and figured out what was there from the shields. Literally in the camp right before that village Genn outright says ‘I don’t know why she was here’.
The fact that Genn did a good thing was more coincidental than anything else. He and Rojers were just out for blood because Varian died and Genn and Rojers need very little motivation to go killing forsaken or even horde in general.