Have you actually beaten him by using X and only X?
Of course you can X mash in the fight, but I highly doubt you can use it and it alone (excluding the Analog Stick which is needed to move) since I know that in KH1, he'll hit you and teleport away, or surround himself in pillars of flames after attacking you, making it impossible to hit him; in KH2, he'll just whack you down to 1HP easily with his sword combo as you run up to him, and then he'll just easily kill you after that.
Unless somebody (most likely Tavrobel) found out some super secret method to X-mash and kill Sephiroth, it's basically impossible to just X-mash to win.