I remembered that Smullyan usually handles this by having an outsider introduce the rules, rather than by having an islander (etc.) who is subject to the rules

I remembered that Smullyan usually handles this by having an outsider introduce the rules, rather than by having an islander (etc.) who is subject to the rules try to explain them. In the movie the goblins themselves are explaining their own behavior, which is not very helpful if you take them fully literally.