I read someone say that “the secret of Monkey Island” is just the fact that you can enter Hell from the giant monkey head, that’s it. In fact the sequel is called “Monkey Island 2 - LeChuck’s Revenge”. No secret whatsoever. Asking what the secret is is like asking what the revenge is.
I mean, even if it were as suspected (“it was all a child’s play”), that wouldn’t be the secret of Monkey Island, that would be the plot twist of the story arc, but has nothing to do with the actual island. At least, that’s my opinion. Stop asking “what’s the secret” and start asking “what the hell was supposed to happen in part 3”.
But we went, guess what?, off topic.