The claim for evidence of mass slaughter of Hindus by Muslims on anything approching such a scale is entirely bogus, conjured up by subtracting a more-or-less plausible recent population total from a highly dubious or entirely invented number pirporting to represent some date in the past. The pioneer seems to have been KS Lal, whose Growth of Muslim population in medieval India (Delhi 1973) concluded without any evidence that India's population had plunged from a wholly arbitrary 200m at the start of the 11th century to a reasonably sensible 125m in the early 16th: Simon Digby in his review of the work (Bulletin of the School of Oriental and African Studies 38:1, Feb 1975) quoted Hobsbawm's description of similar pseudo-computation as "Garbage in, garbage out". The "estimate" cited here seems to derive from an earlier claim of 600m in 1000 attributed (JM Datta, Proportion of Muhammadans in India through centuries, Modern Review, Calcutta, Jan 1948) to the Mughal historian Firishta: even the gung-ho Lal couldn't find any such figure in the alleged source.

The 600m is frankly a ludicrous figure, implying that 11th-century India supported more people than the whole planet half a millennium later, and more than the whole of India and undivided Pakistan into the 1960s. There's really just no reliable figure for Indian population before perhaps the 1590s, when a survey of Akbar's domains has been interpreted as indicating about 145m for the whole subcontinent. Before then, we have multipliers like thirty to scale up from numbers of warriors, etc - hardly a secure foundation for anything much, but not suggestive of 200m, let alone 600m. There is simply no basis for any number of people killed in the subcontinent in the medieval or earlier periods: that doesn't mean slaughter didn't occur, just that claims of x million victims are complete fabrications, which leads one to wonder why the case for mass butchery wasn't considered strong enough without them.