That sounds reasonable. My guess is that he wanted nothing to do with BCN and thus went out on his own. As time goes on, if BBR continues to grow I think it's safe to say that CZ will not remain the sole developer past a certain point. And if for whatever reason he resisted that and wanted to stay in control alone, I believe that would be the end of BBR.
My experience suggests that you're right - I submitted several patches to BBR for the miner and a few typo fixes, etc., and CZ accepted them rapidly and graciously. I didn't get any weird vibe from him about not wanting to let others into the code -- in fact, quite the opposite. For completeness, since this entire discussion just *reeks* of confirmation bias with everybody trying to find evidence that their decision was correct, that mirrors my experience with the XMR crew. Both have been very happy to have help from outside.