You are taking the term "core" too literally compared to the intent of the 4e D&D team. The idea was that everything was "official." There was a movement in prior editions (particularly 3e) to assume anything outside the the big 3 (what you are calling core) was more official: better balanced, more playtested, etc. The "everything is core" idea of 4e was them trying to say that everything will be equally balanced, thought out, playtested as the big 3. I don't think that was necessarily achieved, but I think the idea had merit. I think they could have chosen a better word for you, as you seem to take things very literally, but I think most people understood the intent.