User blog comment:Xilinoc/Ch. 618 - The Dark Arm/@comment-11446176-20150305123333/@comment-4822693-20150305144243

Shunsui has the key to get out of Muken in his chest. Without that one key, they can't leave. I'm guessing that the key would be destroyed if Shunsui dies, but while he's alive it acts kind of like the Oken. He just needs to have it to be able to leave.