We’ve had this discussion before, in the previous Jeopardy thread. (See the surrounding posts too.) I’m aware that Manhattan surrounded by water and thus, technically, an island. Furthermore, I don’t think anyone considers Manhattan not to be an island in the sense that Pluto is not a planet. AFAIK, no Council of International Island-Deciders has decreed that Manhattan is not an island.
What bugs me about this particular writers’ quirk is that they use this factoid to try to throw you. The clue is not testing whether you know that Manhattan is an island. If that were the case, it wouldn’t bother me. If I’m taking a geography quiz and there’s a question on it: “True/False: Manhattan is an island,” I’m going to pick “true.” If there were a Jeopardy clue “The New York borough of Manhattan is one of these, a land mass surrounded by water,” or “This New York borough, home to Wall Street, Central Park, and Broadway, is an island,” fine. What bugs me is when they use it in sort of a trick question sense, as in the clue being discussed in the thread I linked to. It would be like having a clue, “This fruit, native to the Americas, was introduced to Europe by Spanish Conquistadors in the 16th century” and the correct response being “tomato.” Yeah, technically, the tomato is a fruit, but it’s so uncommon to think of it as one that referring to it as such is kind of deliberately deceptive.