User blog comment:Saratje/PC release date explained - Skyrim: Dragonborn DLC/@comment-4973974-20121202020458

This might've made sense, except for these two facts:

1.) Hearthfire had the 30 day exclusivity, as others have already said.

2.) On Bethesda's own website, they list Hearthfire as "DLC". (Go to http://bethsoft.com/en-us/games/dragonborn and scroll all the way down to the bottom of the page. Under "Similar Games" you will see Hearthfire, with the label "DLC" in the corner.)