To people at large, life inside the harem was a mystery. - K. S. Lal

The source is the late Delhi Univ history professor K.S. Lal. But his claim is not that 80 millions were physically killed by the invaders/occupiers, merely that 80 million have gone missing from the extrapolated demographic development (itself already a very risky guess) during that period. This includes people killed, but also the effect of lawlessness and famines which he claims were triggered by the extremely high taxation and other deliberate poverty-promoting policies of the Delhi Sultanate.

The large establishment of wives and servants rendered the nobles immobile. No Indian scholars, engineers or travellers went abroad to learn the skills the Europeans were developing in their countries. While people from Europe were frequently coming to Hindustan, no Indian nobleman could go to the West because he could not live without his harem and he could not take with him his cumbersome harem to countries situated so far away. Europe at this time was forging ahead in science and technology through its Industrial Revolution, but the Mughal elites kept themselves insulated from this great stride because of inertia. Consequently, the country was pulled back from marching with progress, a deficiency which has not been able to be made up until now.

In short, while there can be no doubt about the presence of some Muslims in Sind, Gujarat and on the western coast of India, their number till the end of the tenth century was almost microscopic. In Hindustan proper, east of the river Indus, there were hardly any Musalmans in C.E. 1000.