It is not "confirmation bias" to consistently note that pre-modern numbers are not accurate, are often an order of magnitude to high and thus shouldn't be uncritically taken as 'fact' simply because someone in the past said it. They are not based on any rigorous methodology, just propaganda, hagiography and random guesses repeated over time.
It's the equivalent of someone in 2500 reading a Trump quote and accepting as fact that he had the largest ever inauguration crowd simply because he said it.
Pre-modern literary history wasn't written to be objective 'academic' history in the manner we think of today.
History 'provides evidence' for countess things that are completely fictitious, the purpose of critical history is to try to identify the true from the false.
"History" provides "evidence" that the Persians invaded Greece with 2 million troops. Do you believe that?
"History "provides
lots of "evidence" that the Muslims defeated a force of 400,000 Romans at Yarmuk (it even provides "evidence" that the Muslims were helped by a load of angels). Do you believe that?
"History" provides "evidence" that 36 million people died as a result of the An Lushan Rebellion? Do you believe that?
Why makes you think your numbers are more accurate than these?
Estimated by people who are completely wrong
As I've been saying, the problem is you are basically an order of magnitude too high with everything.
The population of
The World wasn't even 600 million then, let alone India. If you don't believe me:
World population estimates - Wikipedia
Unfortunately the numbers you are relying on have no connection to reality.
It was a link to the historical population of India showing it was 10 times smaller than you assumed.
So if you had said the population of India was zero, you would have been less wrong than if you had said it was 600 million. Just the same as you'd be less wrong if you said zero deaths rather than 80 million.
Do you now see why it is completely implausible that 80 million people were killed, let alone 400 million (which was significantly more than the population of the entire World in 1100)?