The idea that Faerun or dnd as a whole doesent know borders is outright false. Just as the notion that during the middle ages we dident have borders. Are you serious? Ofcourse we had borders. Why does everyone believe that the people in the middle ages were uncultured swines who had no form of common sense, were all illiterate and dident wash.
They had maps with clearly defined borders just like they have in dnd.
Maybe they werent as accurate as maps today are, but the notion that they dident have any at all is laughably false.