BBSV2 is the Mystery Game. The one that's been hinted at for years. He was most likely just saying that we don't know or understand much about the game at this point.
Yea, I thought after BBS he said that there is one game and a mystery title left before KHIII. Those would be 3D and BBS2. Hopefully BBS2 is FINALLY officially confirmed at the TGS as all we have about its existence is the secret movie in BBS, which is just a strong assumption at this point.