You said their attack was motivated by Alliance's interests. Not their view on what that entails. And those are not synonyms in the slightest. Again, the very reason why we needed Class Orders to lead the charge against the Legion was the fact that the Alliance and the Horde were at each others' throats and couldn't do so as they would usually do. That's from Blizzard's own preview of Class Orders at Blizzcon. If the outbreak of the conflict was so dire that it prevented the factions from doing what they'd usually manage, that shows their capabilities were weakened as they were now stretched too thin in two different conflicts at once. That's, once again, the opposite of Alliance's interests. Unless you want to tell me that Alliance strives for being too weak to deal with an ongoing apocalypse, but I find that somewhat doubtful.
But let's look at this perspective of theirs anyway. The idea that the Horde hung the Alliance out to dry at Broken Shore, let alone led to Varian's death, is a pile of nonsense that doesn't hold up to any level of scrutiny. Even from their perspective. When the Alliance retreated from the Broken Shore they did so via gunship. Which provided them with the aerial view of the battlefield. Which, in turn, would have shown them unobstructed view of 1.the Legion spaceships that forced the Horde to retreat (you can still see them even from the Alliance's ground position, but only two), 2. the active Legion portals on Horde's side and 3. the swarm of demons that crawled out of these portals, completely overrunning the Horde position once the spaceships broke their ranks. And since Genn himself spent the flight looking towards the battleground and shouting at clouds, he personally had all of this information available to him from the get go unless he developed some kind of selective blindness. At least the likes of Anduin have an excuse of not being there themselves, Genn has none.
And there's absolutely no causal relation between Horde's retreat and Varian's death. Varian got killed by the Fel Reaver that wasn't even on Azeroth when the Horde left, the gunship arrived immediately afterwards and took care of anti-air and the demons from the Horde's side would need a hike around the entire island to get from there to the Alliance's position unless they wanted to jump off a cliff first. There is however a causal link between the Horde's retreat and non-Varian Alliance members surviving, because they only retreated because the Horde retreated first. If the Horde followed through with the only alternative they had of "dropping dead", the Alliance would have been obliterated as well. Especially since we learned five minutes after returning from Broken Shore that it was a trap (that the factions walked in thanks to Alliance's incompetence no less) and one that was literally unwinnable.