I looked at a recent interview, that had some audio recording issues so the transcript is damaged, but see what it says here.
https://old.reddit.com/r/wow/comment...con_interview/[Question partially inaudible - re: story planning, player reactions and choice]
The big picture arc is definitely laid out well in advance (months/years)
[Re: Tides of Vengeance cinematic] “…Horde players will have a questline where they are tracking down what became of Saurfang, and Alliance players will learn his mysterious escape from -- one of the Horde heroes right in their midst. And try to understand how that happened, how does...the Horde escape from under the nose of the Alliance…”
Even with the holes, it reads clear as day. Anduin is keeping his actions a secret, even from the Alliance. The Alliance will even try investigating how it was possible, if I'm looking at this right.