You said it, _almost never_. The neutrinos coming from JPARK are all emitted as muon type neutrinos. What they are looking for at Super-K are electron type neutrinos. Neutrino oscillations will convert some of the muon neutrinos to electron neutrinos and a very small fraction of these will be seen at Super-K. Based on the number of neutrinos seen, even if it is small, they can estimate the number that oscillated. In this case, they saw 6 events.