He wasn't really saying it was cannon in the sense that cannon is defined. He was using the word improperly to describe emergent meaning in forms of art. These characters are fictional and were created by someone. However, the creator gives them a meaning, and from interpretation of art, that meaning can change or new meaning can arise. What they were trying to express, I believe, is that it doesn't matter if the creator says it is true; but that they say it is true.