The article isn't even focusing on that, AFAICT.

> It's the UCL group's estimate that 60 million people were living across the Americas at the end of the 15th Century (about 10% of the world's total population), and that this was reduced to just five or six million within a hundred years.

The only thing that was that early, that rapid, and that devastating was the diseases introduced by early European visitors that preceded their colonization by decades and leagues.

It's mostly about the introduction of diseases like Smallpox and Malaria that did almost all of the killing. Lots of places like the American South and the Amazon basin lost huge populations without any European having to show up in person.

Are you implying that several hundred of people (guided by Hernán Cortés) killed 5 million of people?

The New Spain conquest (current Mexico) was done with the help of natives that hated Aztecs. Even with the help of gunpowder it would have been almost impossible to achieve that body count. Thus, the Spanish Crown wanted the help of the Catholic Church, so it promises keep the population to christianize them.

Native population still exists in South America while in USA they were almost exterminated and placed in reservations.

What's your point?