It was made clear in the first few pages of this thread that the book is /supposed/ to say this, but doesn't: that it was written by Erik & James, but at some point down the editing line it got cut. Which is a shame, since this one line would have prevented most of this thread, I'll wager. :-( In it's absence, the best you can do is use the book on the understanding that it's supposed to say that.
Basically, it says that this book is going to be the source that all other fiendish topics will refer to: so if a new Dragon article or published adventure or whatever refers to Orcus, this is where it will direct you to go. That's not an unreasonable thing to say: what would the point be in a sourcebook on a topic otherwise?