Sephiroth does not have a Limit Break. This is clear from the Kalm flashback; you can check the menu and see that this is the case. As to Super Nova, I always interpreted it as an enemy-only spell (neither a summon nor a limit). My reasoning behind this is that no enemy (other than Seph, if you count SN) ever uses a summon throughout the course of the game. Further, every summon you as the player controls is some sort of being or group of beings. As to the possibility of SN as a limit, Sephiroth does not use it in response to damage. Instead, it occurs at a certain point in the battle if you have not defeated him yet.