They don't need that much promotion for the last book in such a famous and big selling series, it's mostly store-based, and otherwise it's web-based initiatives by MacMillan's in-house team, some of them like releasing chapters and such. At this point, the campaign is probably ready to go anyway (they would have done all they could in advance, with the book coming out so early after the Christmas vacations). And they no longer do ARCs and all for WOT. All in all, it's pretty much the kind of book that sells itself.

Let's not forget Tor wanted to release it before Christmas, it's Harriet who insisted to move it to the latest available date of the "season" not to feel pressure to finish. Now it sounds like she was able to complete her work sooner (last we heard, they were proofreading, which means the final text is established and they're checking for last mistakes and errors in the galleys - missing words or paragraphs and such). It wouldn't be surprising if Tor (and Harriet) reverted to their more ideal release month if the book is ready.

I tend to agree with Terez below that it could come much sooner than we think (like end of October, early november). That could be convenient for Brandon too, it would let him do the book tour before his wife gives birth.