I thought he was referring to the disparate elements within a single theme - so links between the novels, adventures, MMO content, etc that all come from the same story bible. I could be wrong though.
I copied out his whole last comment about sourcebooks: "One of our creative challenges is to package [setting] material - reintroduce facts and important details about our worlds - in a way that we know that DMs and players are going to use, that's going to excite them, that's actually going to surprise them. We may get that content out, but I'm not going to guarantee it's going to be a book. I'm not going to guarantee that it's going to be anything that you've seen before. But it will be something."