From what I understand, this content was largely already done, and was in the beta for at least a little while.
Also, this patch fixes a lot of bugs (impressive list, in my opinion), not just adding new features. The patch notes are on the forums.
I'm guessing they have a massive team with many people fixing bugs, but others still creating content. In 3 months when there isn't a content patch, people will be complaining about no new content in their bug free game, if everyone was dedicated to fixing bugs.