I answered that question, and it's because he wasn't Seifer Sephirtoh until his union with Jenova's head. Before that he was just Sephiroth trying to use Meteor against the planet. As Seifer Sephiroth he's gained new attacks, one of which are Supernova. He obviously did not perform the attack in canon however since well, it would've destroyed the solar system while considering that none of the party killing overdrives (such as the Final Aeon's Ultimate Jecht Shot from FFX) were pulled off, since it would've obviously killed the party thereby creating a bad ending. However Supernova is still an attack of his and can be performed as shown in the game. I'm not convinced at all of this illusion theory. I'd have to believe that just about all the overdrives and summons are illusions as well if I were to believe Supernova is an illusion.
Asking why the planets appear again while doing Supernova is exactly like asking why the FFVII party are still alive after taking a Terra Flare from Bahamut, and why the planet Eden nukes seems to be perfectly fine right before she nukes it again. All of these are the bs mechanics once again, not the summons themselves.
As I've said before, there's no exact proof on either side of the Supernova debate. Why this is still being argued is beyond me.
Originally posted by Guilty Gear
Dammit Nomura.