First of all, whoever said Brits started casteism has his head buried deep in sand. It is quite true that they did use it in their 'divide n rule' policy and exploited it wherever it suited them, but they definitely did not start it , IMHO. and they did use it as an excuse as to why India has to be de-sanskriticed. Macaulay had a very interseting take on how this could be used to create a subclass within india, Indian in looks but servile to the British.

As for the people from Andaman, do take a look at the map. Its an island in the Bay of Bengal and hence far removed from the mainland. Inbreeding doesnt need caste system there.

Also if you see the typical agrarian society, they do not move. Moving would have meant crossing quite a lot of geographical barriers which is not something farmers generally do.

They have a point on DNA validating caste system if samples from the same site showed those distinct races. I do not see any mention towards that, here.

"The first was genetically close to people from the Middle East, Central Asia and Europe, while the second had an 'Ancestral South Indian' lineage confined to the subcontinent. "- That would be the Aryans and Dravidians, I suppose. still there are quite a few historians who still claim that the 'Aryan invasion' is a myth'