Kenpachis easy defeat makes me call into question the canon-icity of that novel that pits him against a former Kenpachi arguably stronger than Aizen. I've recently read about some other crazy stuff with that series though, like
It's essentially expanded universe stuff, and this last chapter convinced me that we won't be seeing the books interacting with the manga in any way. Basically it was just a moneygrab. Hired some writers and let them go wild with it.