That's a mistranslation (or rather, a misleading translation); Kensei never says that Hisagi can currently (as of that chapter) use bankai. A closer translation is more clear that he's telling Hisagi to use bankai despite him not having it (IE, he's ordering Hisagi to obtain it and show him), and he was going to have Mashiro beat the crap out of Hisagi in an attempt to force it out of him.
XI Wiki



