That’s why I began my post “In folklore studies…” It’s not wrong to use “myth” to mean “untrue fact” or “false belief” in ordinary speech or other fields of study. I only brought it up here because 1) that’s the field of study most directly relevant to the OP and 2) this is a board dedicated to fighting ignorance.
By the way, the etymology of the words is not 100% helpful: in Greek, mythos has a much wider range of meaning than it does in folklore studies, and Latin legenda and superstitio likewise.