If I recall correctly, the original Warcraft 1 storyline is canon and the Horde sacks Stormwind.
Also, the original Warcraft 2 storyline is canon for the most part as well, first it follows the Horde storyline (except for the last mission or perhaps last 2 missions, in which Lordaeron is conquered) and then follows the complete Alliance storyline.
So what I was saying is true, except if they retconned it with the books that follow the Warcraft 2 storyline, I never read those. They were only written a few years ago.