Take these stories with a grain of salt though I don't doubt he said this.
He's speaking rubbish now? I hope. Unbelievable. If one goes to his wikipedia bio; one does see though, that he had Catholic schooling when he was young. So, I hope he's not a Muslim, I thought he was "unaffiliated" because he had a bad experience in the Catholic Church, see his bio.
Nope, it's one of the few things I've heard him say that I have 100% confidence that it is true. He was mayor of Davao, on the island of Mindanao. Whereas the rest of the Philippines is 95%+ Catholic Mindanao is about 50/50 and some parts are 90% Islamic. The Island is extremely dangerous for Americans. It's a safe bet that much of Duterte's antagonism to the US is founded in his Islamic background...