At the end of the day the decision of what counts as a myth is up to the voters. If you can convince them that what you have mapped illustrates a myth - however you choose to define that term - then you should be fine. If instead you map something and people decide that it is not a map of a myth then you'll lose votes. I'd suggest that you make sure that your map makes the case well. In my own opinion, the mythical locations of purgatory on earth would certainly count, but my vote is only one vote.
The rules allow us to disqualify a map if it does not fit the challenge parameters but I don't think that has ever happened. Instead the votes usually enforce the rules pretty well.