Well you argued 400 was plausible because you assumed the population of India was 600 million instead of 70 million. When presented with evidence of this error, you strangely enough didn't think to question if your original "conservative" estimate of 80 million might also have been equally fantastical.
You can lead a horse to water and all that...
Repeating the word "strawman" without comment is not an argument.
As for your numbers, the logical place to start would be why do you actually think that is true in the first place? It's not like you've presented any evidence other than a list copied off reddit and something else that claimed the population of India was higher than that of the entire world.
How Many People Have Been Killed in the Name of Religion? : atheism
Even though you obviously have not looked at the issue with any degree of scepticism, hence your elementary errors, you still assert it as unquestionable fact as it is "not impossible".
As for 'cannot be refuted' or 'not impossible', it's like saying prove Jesus didn't rise from the dead or walk on water, you can't do this, you just have to use your brain. It's a Kent Hovind challenge. I can't objectively 'prove' that the Persians didn't invade with Greece with 2 million troops, that doesn't make it any less ridiculous.
As to your claim, it's pretty obvious when you look at the demography and economy:
Country X has experienced 4 centuries of great economic growth and has undergone a population increase similar to the rest of the world over this time.
At the same time, this county has experienced 'the greatest genocide of all time', a 400 year long exposure to violence 'conservatively' estimated at the level of the 30 Years War which devastated Europe.
Alternatively, we can say the equivalent of a country that lost people to violence at the same rate as Nazi Germany during the industrialised slaughter of WW2 (including the holocaust in Germany) for an entire 4 centuries is thriving. This is a country in which, on average, around 15-25% of people who reach adulthood would die a violent death.
Unfortunately, the only evidence we have to support the idea the greatest genocide in history happened this way is a collection of sources that we know, for a fact, are making up numbers out of thin air and have neither the ability or desire to craft numbers based on an objective, robust scientific methodology. We know, for a fact, that all such sources across all such cultures faced this same issue.
In light of incomplete information we are left with inaccurate literary sources which generally serve as propaganda or hagiography (as all 'history' did back then), versus basic logic, empirical experience and what we know about the realities of running historical empires.
As such, when being presented with this information, it seems most reasonable to assume:
a) It probably happened
b) It probably didn't happen
(Another thing would be to consider the very concept itself "Muslim invasions". There is no reason to treat these as a single event, any more than an Indian historian should treat the Napoleonic wars to WW2 as the "Germanic Invasions", call them the greatest genocide in history and say 250 million people died during them. It's the historical equivalent of saying "well, they all look the same..." In European history, the equivalent of a Mongol (Timur) invading a Muslim Sultanate that had existed for over a century would not be considered as part of the 'same thing' as the Turko-Persian invasion which formed that Sultanate)