So, Romila Thapar is not a Dalit activist, just a historian (I'm guessing this is a source of confusion; I could be wrong).

I'm saying they should have read up before starting their project.

I can't find the study for some reason, so I'll try and do it from memory. They randomly picked from a city Dalits (Dalit is a catch-all term coined by B R Ambedkar for people of the lowest castes, and people outside the caste system, all of whom were treated horribly) and people from the merchant castes to look for genetic differences. Which is all fine and dandy - but for the fact that neither 'Dalit' not 'merchant-caste' is an actual caste; there are many castes which come into those two categories. So, assuming a simple no-inter-caste-marriage model of caste, a merchant family from village A thousands of kilometres from village B has about as much (or, considering marginal things like babies born out of rape, even less) genetic material in common than a merchant and Dalit family from the same village - unless there's a common genetic ancestor to all merchant families. And that's where reading historical literature comes in - the history of caste is much more complicated, involving for example periods when it was barely enforced and shuffling and all sorts of stuff. So, they will find differences in their study, but it won't reflect actual caste differences.