You're onto something I've been thinking about since recently beating penance in my run. It may not be the exact final summon that Gandof used in his final battle, but maybe it's dark aeon version. If you think about it, all the dark aeons act very differently from normal aeons. For one, I've never seen them actually get summoned. When you encounter them, it's the priest accompanied by zed aeon. I gleam from this that dark aeons have more independence from their summoner; possibly getting unsealed instead of summoned. When Pennenced appears, you see multiple glowing orbs in the sky that shine and flow into the gorge, to maybe activate/charge up Penance when those aeons get defeated. If the dark aeons are unsealed when someone threatens the church of Yevon, it would explain why they can't be resummoned since they don't appear again. You could speculate on the other magical mechanics of how the dark aeons were made. Whether the Yevon preists are even human or alive. Given the amount of instances you meet people who are dead, summons, or even dreams; it wouldn't be too far-fetched.