The king was mythical (there are historians who believe he existed, but its so old I would rather call it mythical), but the concept wasn't. You will find the references in quite a few scriptures that date back thousands of years. End of the day, what matters is that, whether the king existed or not, the people in the subcontinent had the concept. Concepts of 'promised land's and land belonging to a particular ethnicity is nothing new, right?