The article doesn't really state what your short summary states.
The reason Polymarket bettors got the papal conclave result so wrong is that the event is extremely hard to predict, Domer, one of Polymarket’s top pseudonymous bettors, said on X.
“It's like walking into a store that doesn't communicate with the outside world,” he said. “Not even the participants themselves would probably know how to handicap it.”
Choosing a new pope isn't an open process, so there's very little information to go off of. Something like an election has a lot of public information, so those betting odds are more likely to represent the actual odds in the election.
I really don't think there's much to learn here, other than that choosing a new pope is chaotic and the process isn't very open.