Did Ambedkar say ancient Indians ate beef?
He did, in a 1948 book, and built a historical argument on it. In The Untouchables he contended that beef-eating was once common in India, that even Brahmins ate it in the earliest period, and that the later prohibition was a much more recent development than popular belief assumes. This was not offered as provocation but as evidence for a larger claim about where untouchability came from.
His thesis was that the origin of untouchability lay in a religious struggle. As orthodox practice turned against the killing of the cow and made abstention a mark of high status, the communities that continued to eat the flesh of the dead cow — because they were the ones who removed the carcasses — were pushed to the bottom and eventually below the caste system entirely. Untouchability, on this reading, was manufactured by a change in dietary rules, not handed down from the beginning of time.