Myth: Shyarly only said the Destroyer of Fishman Island would be wearing a Straw Hat, not that it is Luffy. Jinbe’s going to go get a bunch of Straw Hats for the children of Fishman Island. Whoever the Destroyer is will just take one of these hats to taunt the Fishmen and/or the Straw Hat Pirates. After all, Luffy would never destroy Fishman Island! Plus the Straw Hats have left Fishman Island, and so far they have not gone back to any previously visited island!
Fact: So far, nothing has been confirmed. While Keimi commented that Shyarly’s prediction could take up to a year to come to pass, there was no direct evidence that Luffy is indeed the Destroyer. While the Straw Hats have not yet backtracked, with the introduction of the three needle Log Pose, the dangers of the New World, and the Straw Hats’ promise to Shirahoshi may mean that the Straw Hats will revisit Fishman Island before the end of the story. However, this myth may just be hopeful thinking by fans, and the circumstances under which Fishman Island will be destroyed, if it actually is, are unknown.