They might have jurisdiction to investigate that time frame, but they no longer have jurisdiction to do so from inside the Philippines. It makes a difference. They also now lack the power to punish him. Regardless the Philippine people love the guy and the majority of them support his policy. Isn't he now running for vice president of the Philippines? Think of it as when Medvedev became president of Russia and Putin became PM. Hate to say, but the guy is going to skate on this.
In order to investigate a country, it has to be a member/signatory to the ICC. Phils was but when the ICC announced that it was investigating Duterte Harry, they withdrew from the org. But the ICC has jurisdiction to investigate all allegations made against him prior to that withdrawal.