Well you are right that, if we extend the argument made by the findings at Rakhigarhi, that IVC was predominantly a society made up of Dravidian speaking people whose present related group are settled in very south of India. The argument the migration school will make is this:
a) That it strengthens the old argument that north India was predominantly settled by Dravidian speaking people. Who were later pushed south by an Indo-Aryan speaking population. The presence of Dravidian substratum in the early rig vedic Sanskrit is cited as the evidence of this early interaction( but a subsequent isolation)
b)The evidence for the above argument is that the present predominantly Indo-Aryan speaking population has genetic affinity with populations in the steppes and the Europe, but this genetic affinity is not shared by the predominantly Dravidian speaking populations in south India. If the Indo-Aryan speakers and Dravidian speakers were once from a common stock, then the Dravidian speakers would have shared genetic traits with the Europeans and the steppe people like the Indo-Aryan speakers do. So the argument is Indo-Aryans are an intruding group, while the Dravidian speakers were, relatively speaking, indigenous.
P:S - Nair affinities with north indian social groups is not very surprising, considering that there is a popular theory that they were one of the groups, along with the Namboodiri Brahmins , who migrated from Ganga basin to the Tulu speaking areas in Canara areas in Karnataka and later moved into proper Kerala. Even if they as a social group was indigenous to Kerala, their intense Marital relationship with the Namboodiris in the historical period, would have skewed the genetic affinities wiuth the intruding groups from the north.
