They told him that eating cow meat was allowed before the "Bharata war" and the age of vAsudeva, but has been forbidden since
But from his account there is little doubt that the Bharata war and VAsudeva were key figures - and the War changed many things in Indian life
Al Biruni says something intriguing. He says the bodies were exposed to the elements in the Zoroastrian way by Hindus as well
But post nArAyaNa (who I presume is vAsudEva), the custom of burning bodies picked up
Even the tradition concurs on this - that things changed with the end of the DwApara Yuga and the birth of Kali - which coincides with the conclusion of the bharata War
But what was it in its origins?
A Satvata text which gained status in the Vedantic fold over time?
That's something that deserves a closer look