tengoyama hitetsu said in the last chapter that the sky is clear, so there's no way that the people in the flower capital won't notice a giant island flying in their direction, someone will definitely see it and warn the people giving them enough time to escape.
kaido doesn't want to destroy onigashima so he won't drop it that hard, he will carefully land it on top of the kozuki castle, which means there won't be any massive shock-wave that can destroy everything in the flower capital.