Low Myth, or what is apparently now being called 'Tabula Rasa' (blank slate for those who speak English and don't want to sound fancy) means there is no established fiction at the start of play, or at least the established fiction is pretty general and significantly 'open'. A DW game, for instance, is ideally starting with a true blank slate. The participants get together and have a Session Zero, before that NOTHING exists, there is no fiction, only presumably the fact that Dungeon World will be played, and therefor its tropes and genre will shape play."Low myth"? That's a new term. Okay.
There's no reason why that can't happen, but I certainly agree that is not commonplace or the expectation.
The point being, in such a game, the sort of conversation that @chaochou outlined is ESTABLISHING, it is the initial creation of the fiction, there's no setting out there that this stuff fits into, no map that the GM is going to consult and decide "Oh, OK there's a village over here on the edge of a forest, so to the north must be the Orc Badlands, and the Dismal Dank is south of there, and the bad king must be the King of Farlong. Nope, there's nothing! The way things are, what is true, the facts are all going to come into being as the players and the GM explore, ask questions, solve problems, etc.
One HUGE fundamental consequence of this is that the GM doesn't get to say "Nope, you can't find a pirate treasure map that shows Treasure Island on the South Sea, because there is no ocean to the south! Actually, there is! I mean, unless it clashes with some other fiction already established in play, obviously.