I'd say "If you fail to uphold honor" is pretty vague. But it wasn't my main point anyway.
I'll definitely take Wrathion's word as truth, it's not like he failed any plans or anything.
aka Varian does not, did not, could not have the "power to defeat the Horde."
No one does. Just as no one has the power to defeat the Alliance.
Both factions include some of the most powerful mortals and countless regular folk in them.
You realize that's exactly what a horn is, right? If Sylvanas wanted to be sneaky, she could've made the escape just visible to the Horde as they were on higher ground. She warned everyone with the horn.
And it wasn't the same thing for the Horde, because while Sylvanas blew the horns of retreat when she realized it wasn't going to work, Varian was yelling "we finish this now" when he saw Gul'dan summoning a dozen of the most powerful demons.