The background to this notion is the genocide supposedly committed against the native inhabitants of the Americas.
And indeed by 1600 the native population had been reduced by in the neighborhood of 90% from what it was in 1492.
The problem with the theory that this constitutes genocide is that 90%+ of the death toll was the consequence of the merging of the Afro-Eurasian and American disease ecologies. The vast majority of those who died never saw a white guy.
The death toll would have been exactly the same had an Aztec ship sailed up the Thames and “discovered” Europe.
Actually, as you probably know, many Europeans during the middle ages were killed by mostly the same methods. A number of historians believe the various plagues like the Black Death and bubonic plagues that killed millions of Europeans were brought in from the east or other places outside Europe.