User blog comment:Kaido King of the Beasts/Top 10 GOOD Things About the Blood War Arc/@comment-29359011-20160827030619/@comment-28044177-20160828061423

If Yachiru was in any way in Zanpakutou Spirit, then that would explain how he obtained Bankai. In order to achieve Bankai, you need to be able to materialize your Zanpakutou Spirit in the real world and subjugate it. Zaraki would have been doing that for years.