During the Fishman Island Arc, Madam Shyarly's in Coral hill predicted that a man with a straw hat is destined to destroy the Fishman Island. I can't think of any other besides Luffy that is wearing that hat. How can Luffy possibly do it? because in the first place he saved that island and he's not that kind of guy that will do such thing.

Fishman island lies right underneath the red line and Mariejois. So destroying Fishman island means destroying the capital of World Government, Mariejoris.

They say Madam Shyarly's prediction never failed. So could this be another straw hat guy? or her prediction is just wrong?

Questions:
-How can Luffy possibly do it? (if he is the destined one else)
-Could this be another straw hat guy? or her prediction is just wrong?