If there is one thing I have learned about Blizzard, things are not always as they seem.
From the pictures that have been shown and things they have said, it does seem that Sylvanas uses piece of Sargeras sword to burn down Teldrassil. Along with the prologue of the book being shown about her talking about Stormwind, it sure implies the Horde started it. But I can't help but think that's not all there is to it, and that new council that has formed in the Undercity has something very sneaky to do with it.