Maybe not by definition, but it's almost irrelevant because a not anonymous source is so much easier to verify / disprove. Sure, in theory an anonymous source could be 100% accurate, but since they are very tricky to back up, they almost always have the effect of stirring the pot without resolving anything, and thus quite often appear as nothing more than attempts to do just that. And after a while, people unsurprisingly start to treat them as such.