> generations of segregated reproduction have selected for higher IQ in the higher castes

There is no natural selection going on here. People are born into a caste and, for all intents and purposes, their caste identity is fixed for the rest of their lives. The idea that these people have somehow developed a "higher IQ" over "generations" simply by being born to a particular caste makes no sense.

Furthermore, the arguments made in the article have nothing to do with poor performance, (possibly) correlating with a lower IQ. The article details specific examples of antisocial behavior by upper caste Indian workers against their lower caste peers.