I see speculation that they got ISIS help; I see that ISIS folk are celebrating; I see suspicions that the attack was ISIS inspired; I read of fears that the attacks were ISIS linked.
But everybody arrested or detained so far (as of this morning, at least) was Sri Lankan and (as of this morning) there was no evidence that they'd identified any travelers in or out of Sri Lanka for training, or even any communication.
In fact, last I heard there was suspicion that a certain group was responsible but nobody had claimed responsibility.
On edit: It also pays to understand what "defeated" typically means when you're fighting somebody with organized fighters and territory. It means the organized fighters are routed and denied territory that they control. Nobody said that ISIS as an organization was defeated; in fact, the suspicion is that driving them from a defensible territory will, like German werewolves after WWII, drive them to terrorist or possibly even insurgent tactics. Few argue that the Germans were not defeated in 1947, but a few years later when the werewolves ended. Or, worse, for the sake of propaganda, saying that since there are still groups with Nazi-oriented ideology, WWII is still ongoing.