User blog comment:Xilinoc/Ch. 564 (Mask is too fabulous to live)/@comment-24113610-20140117070855/@comment-5398287-20140118004342

I would imagine that he knew, what with the relationship between a Shinigami and their Zanpakuto being so intimate. However, he probably didn't find it worth mentioning, since it hadn't been noticeably changed (there were still a lot of segments left over) and appeared to be just as powerful regardless.

As for his not knowing Hihiou Zabimaru wasn't his real Bankai, it's not too unlikely. If you'll recall, a Shinigami has to combat their Zanpakuto spirit in order to learn new abilities for Shikai and Bankai, since they don't come immediately upon attaining either stage: with that in mind, it seems entirely possible for a Zanpakuto spirit to grant their master a false Bankai if they don't think they're worthy enough.