Xiamen, Hongkong and Montreal are highly doubtful. In none of theese cities, more than 2-2.5 million are living on islands, the rest on the mainlands. Also half of the population of New York greater area is located on the mainland.
I agree with you on New York City and Hong Kong. But for Xiamen and Montreal you can tell that the cities' city proper limits are clearly only located on the island. Yes, most of the greater area is located on the mainland, but the city itself is located on the island, and I'm not going to make this quiz use city proper population.