Watch the Hearthfire and Dawnguard trailers (the ones put out by Bethesda, not your favored video game news website). They both have Xbox stuff at the end. Dragonborn does not, which indicates to me that it's not covered under the agreement.
As for why the posts only say stuff about Xbox, that's because Bethesda pretty much has to set a release date because Microsoft has to take a direct hand in the process. With Steam, Bethesda pretty much has much more direct control.
Let's also take one last fact into account: they never announced release dates for Dawnguard and Hearthfire. We had no clue when they were coming out until the days they were released.
So before jumping down Bethesda's throat, let's all just calm down and see what happens. Assume it's coming out on the 4th (there's no reason it wouldn't be, especially with the holiday season approaching). Give them the benefit of the doubt.