The site claims there were once 20 to 100 million Native Americans before some unexplained "plague". I've never seen any estimate higher than 20 million. Lots of sweeping claims are made with little support for the claims.
A lot of the blather is building to the assertion that Europeans only defeated a 90% decimated population, and could never have defeated the pre-plague population.
Whatever, once Europeans began settling in the New World, they settled a few hundred here and there and were still surrounded by thousands of natives. The Powhatans were still all around Jamestown in substantial numbers, many times the population of the small Jamestown settlement.
Those writing on that site have an agenda, and it's mostly to denigrate and minimize any accomplishments of Europeans.
It was disease what ultimately killed most Indians off. Dropped them right and left.
The claim of 100 million is if you count all of the Americas, both North and South, and even that’s debatable. Consensus nowadays seems to be 50 to 55 million throughout both the North and South American landmasses.
The 100 million figure is considered sort of out there or ‘fringe’.