Just because some made up stuff has been around a long time, doesn’t automatically make it better than stuff that was made up more recently. And thus XKCD tackles the widely-held but mistaken idea that people in Columbus’ time typically thought the world was flat, while Columbus was a lone voice insisting that it was round.
But while it is bad to make stuff up and then claim it is history, or science, making stuff up is not itself bad. Indeed, sometimes it is wonderful, and the Tolkien allusion in the comic suggests that its creator would agree.
Myths and fantasies are only problematic when we don’t realize what they are, even though we could and should.