The point is, if there is one legion, there must be only one real timeline, as the Titans told Nozdormu, because else the Legion should turn its attention to all the possible realities. Despite what Machazzar said (all realities, all dimensions are open to me) it makes little sense a Kiljaeden that appears to infinite Nez'ruls (or even worse, to a different orc every time) to organize the destruction of the Dranei. A scapegoat could be that while the legion is able to move across different worlds (or timelines), they cannot access them directly unless something specific happens that causes these worlds to diverge from the original timeline, so the Legion actions are always the same, but they can move back in time or in the future when somethings disturbs the timeline.
The actions of Garrosh and Kairos have generated a massive diversion from the original timeline and this has allowed the legion to send their agents en masse from the future, something that they would (or could, as it was only the titan Aman'Thul to have power over time and gave it to Nozdormu to prevent the generation of alternate timelines) not do if the timeline would have stayed the same.
Damn I hate time travel.