How did Blackbeard knew about the level 6 prisioners if the WG didn't told anybody about the priosioners that where sent to level 6?!?
Maybe the drillman told him abou it, if the theory of him being the the one that made level 5.5.
Black Beard has been sailing for over 20 years. Those guys could have gotten captured at any time between now then. And rumors go around about there being more than 5 floors, as Buggy showed.