That "the campaign is initially played solo" makes it sound to me like "your first mission is solo," which it is in the pre-tutorial section of the campaign. I would not be at all surprised if, once you get past the tutorial section, you then can play with your friends. If I recall, Destiny's first mission, prior to the tower, would not let you matchmake either.
I can see how people would read it as "the campaign is played solo the first time," but it sure seems like they're saying the first mission is solo, which, based on the beta, it is, since the game seems to start right up and launch into the beta before anything else.