So basically the Horde did not do it, but it's their fault regardless.Or Blizz figured that all that is remembered and not rehashed as it would be tedious. The fact that people apparently missed all that though is likely part of why they feel they have to do "war is bad" stories.
Saying that the Horde's actions had nothing to do with it is absurd. Even under the most generous assumptions, Varian was inclined to believe the Twilight Hammer actions as something the Horde would do based on his knowledge and experience with them. It is far more likely and believable that Varian, who had absolutely no reason to think well of the Horde, would have fully considered them. Thus, whatever the case, that then means those actions do in fact contribute. This shouldn't need to be spelled out by Blizz.