. . . In which sense “that’s a myth” is, as I said, a well established and perfectly serviceable way to say that. “Urban legend” is more recent, more specific, jargon, that is not applicable in all the various and varied situations you find it applied.
4. A fictitious story, person, or thing: “German artillery superiority on the Western Front was a myth” (Leon Wolff).
You may choose to use them interchangeably, which is probably covered by the gray areas, but that certainly does not make me “completely and utterly wrong.” But thanks for playing. (And thanks for your Snopes reference, but that deals with a different definition of the word. I was not talking about capital-M Myth, as in folklore, but the more mundane usage, as in my example above.)
Please feel free to open a debate thread if you want to try to nail this down more definitively.