There's a whole industry of this stuff, taking isolated atrocities and erecting them into the routine fare of past centuries. Human history is sadly punctuated with such brutal episodes, often inflated for intimidatory or dramatic effect by boastful conquerors or chroniclers, and India was no exception. Even then, it's worth noting that the excesses enumerated in the wikipedia piece total around two million deaths over 600 years, even taking the claims at face value - a terrible loss of life if the figures are to be believed, but not "the biggest holocaust in world history".

I suspected KS Lal would figure in this, and sure enough there he is with his Hindu population decline of "80 million between 1000 AD and 1525 AD, an extermination unparalleled in World history". Lal in his Growth of Muslim population in medieval India (Delhi 1973) derived his number by deducting a surprisingly sensible Indian population of 125 million in the early 16th century from a wholly invented and extremely improbable 200m at the start of the 11th century: Simon Digby in his review of the work (Bulletin of the School of Oriental and African Studies 38:1, Feb 1975) was reminded of Eric Hobsbawm's dismissal of similar pseudo-computation as a case of "Garbage in, garbage out".

Many Indians doubtless died in invasions, rebellions and massacres over the past millennium, but there's no reliable way of counting the victims and no reason to assume any total approaching the more lurid claims. The uncritical and communally charged aggregation and re-telling of these gruesome and doubtless often exaggerated tales coupled with regurgitation of entirely fabricated totals serves no purpose other than to stoke continuing sectarian division.