I think Del Murder was being sarcastic.

From FAQs in this forum:

Q) Is it possible to get Sephiroth as a playable character?
A) The closest to that the game has to offer is when you are at Kalm and Cloud is telling his story. Cloud and Sephiroth fight together but you can only control Cloud during the battles.
I think people just make up these silly rumours to make you waste a lot of your time xD Get over it people, Sephy is just the bad guy, Aeris (SPOILER)is never coming back, and so on. Now..go play FFX-2 or something! ^_~