Per the roster, there may already be a quorum of Cultists (and they probably don’t all know each others’ identities yet).

as this seems uninteresting, whether it hits a group of clever or unaware Cultists. And (if your “Cultists are all as excited about the word ‘win’ as Cuddlebeam is, and will do anything to get there” theory is correct) that quorum of ex-Cultists could just propose that Cthulhu rises anyway.

To me (personally), if there’s a quorum-cult but that just chooses to not play optimally because of their own subjective measure, it would feel like if I’m playing Chess and have gotten legitimately checkmated but the opponent just chooses to move out of the checkmate to “continue the game” because they just didn’t like the checkmate. I’d be a bit annoyed lol, if I’m beat, I’m beat, if I win, I win.

How should cultists ‘optimally’ identify each other?

I you’ve “gotten legitimately checkmated” then your opponent doesn’t get to move…
A closer simile would be a game of mafia or werewolves where the wolves didn’t realize they already had a majority, or choose to keep playing because they feel it’s more rewarding to win without revealing themselves.
In such a situation, if the wolves overlook something and end up losing the game, no one is going to say “hey, no, the wolves had already won”.

In a game of chess, if your opponent has a much better position and you feel like you’re losing, but suddenly they make a big mistake and you capture their queen, would you be annoyed and say you had already lost?

@Koen: That is a fair comparison. My flaw was to reveal that the quorum-cult was likely, but I kinda went straight to grabbing a “Aha - I told you so!” moment for if it did happen because that was what I thought of first.