His rule is "Extraordinary claims require extraordinary evidence."

Three possible claims for Enceladus:

There is life there
There may be life there
There is no life there

Given the very little we know about the plumes, we can't support either the first or third claim. The second is possible. As to whether the plumes are liquid water geysers or some other geologic process, we don't really know. If one possibility had the overwhelming weight of scientific evidence and theory behind it, then the other possibility is an extraordinary claim. That's not the case here though, where all we have is several observations from a spacecraft not designed to answer this question, and just two-three years of theorizing about it.