Shinji would have said "Bankai: (Something) Sakanade" or "Bankai: Sakanade (Something)" if he were indeed using his Bankai. Every person who's ever used Bankai has done this at least once. With the Shikai, it's "Release Command: Name of the sword". Shinji follows that formula perfectly (Collapse: Sakanade). When he uses his Bankai, it won't be a mystery. He'll come out and say it. So I, for one, am certain Shinji is/was only using his Shikai.