Perhaps the Munda-speakers ancestors were there first, and the Dravidian-speakers’ ancestors had Dravidian imposed on them by a small number of conquerors. Compare how Spanish became the first language of large parts of Latin America where the people are mostly descended from the pre-1492 population, or how Haitians speak a French creole despite being almost entirely of African ancestry—DNA testing would no doubt show much closer affinities to people speaking unrelated languages in West Africa than to people in France.