I know what bungie called it. And I think that while its not correct it makes sense if they were marketing destiny as a mmo like game for them to call it that. Secondly because the hive are a threat in the story. And pointed out to be (or at least implied) to be a bigger fear then the fallen. We were given a new character that the dlc revolved around. The story missions pertained to her and her goals. Also because we have blades of Croat invading earth it strikes the presence that this Croat thing is important and here. The over arching story of destiny is about the several threats that you as a guardian must face to protect civilization. Hive are apart of this threat. By making a real character and making the hive threat seem real they expand on the story of the universe some. A side mission to me is the random tower missions you get from becons. And a few of the strikes. This dlc had a strike for everyone about omnigul. Since she is apart of story/ quest lines and the overall threat it didn't feel like an extra piece of content slapped in.