In general, genetic differentiation in populations of India is low (0.26-1.7%), but overall genetic differentiation in 18 mixed populations of India is higher (2.23%), similar to the largest single study on 16 tribal groups from central India (2.18%).
Genetic diversity is actually not that high in India. There have been genetic sequencing projects in Indian groups (look up Dr. Thangaraj's publications from CCMB), and Indian caste groups have low diversity due to strict endogamy.
https://pubmed.ncbi.nlm.nih.gov/8908794/