@darius I did! I saw it in game, though not until I learned about it online because it's so buried—beat the game, then load your clear file and beat the 'true' final boss (which I guess has its own unlock criteria), and then load your clear file _again_, and then watch/skip past the end credits…yikes!
But yeah I dunno what's going on there. The music threw me. Is the Grandmaster Aidios?
@danbruno ohhhh haha I only beat the true final boss. Which btw was really weird structure-wise with how they handled the whole thing? I almost wonder if it's a commentary on the "possible worlds" that Campanella mentions.
The consensus among fans seems to be that Grandmaster is Aidios, yes. Although she says she's like a minor echo of herself. I guess this means we will eventually fight the main church's god like in all JRPGs.