Even assuming this whole narrative is correct, the Horde was going to die against all the hell that was released upon them if they didn't retreat. And once the Horde was massacred for good, the Alliance would have been next.
Except Alliance and Horde rebellion fought together to dismantle Garrosh's True Horde, as actual allies. Outright attacking an ally without warning ("seize this moment Varian, dismantle the Horde!") is indeed backstabbing. The fact that you feel entitled to backstab doesn't change what it is.Retaliating to the bombing of Theramore wouldn't even be close to a backstabbing. The horde didn't do anything to help the Alliance, and the Alliance didn't and doesn't owe them anything.
Baine Bloodhoof says hi. He's basically the Horde Anduin.Why else do you think Anduin Wrynn is the character currently embodying the "End the cycle of hatred" ideal? Because the horde are not even considering it.