From what I gather, it was originally slated for a big marketing push but then Square-Enix broke rank by outing the PS4/PC versions and putting up the pre-orders. When that happened, the relationship 'soured' and the push got downgraded to a whimper.
At least, that's what I understood from Kagari's posts.
That is what I figured happened. I wouldn't be surprised if SE broke the contract by officially confirming multiplat release dates and the result is that MS pulled almost all marketing funds, but kept the exclusivity in return for not suing. I can't imagine a world where MS would have agreed to let SE publicly gut the exclusivity deal before the game would launch.
I really hope the game has legs on the XB and gets solid numbers on PC/PS4, because it is an amazing game.