Ryuma was said to have defended Wano from World Nobles, which only existed after Joyboy/the Ancient Kingdom were defeated and the World Government was established.
Also, Ryuma's grave is in new Wano, not old Wano before the walls around Wano were made, and one of the One Piece magazines explicitly says that Ryuma died four hundred years ago.
All evidence points to Ryuma being around after the void century.