There are some history writers who would agree; 25 million is probably on the low side. I agree, climate changes moved or wiped out large numbers.
If these estimates are correct, central and south America had a larger and healthier population than most of Europe.
It would be interesting to see a graph showing the European and native populations from 1510-1800.
There is an interesting book, “1491 New Revelations of the Americas before Columbus” by Charles C. Mann, which discusses the pre-Columbian populations in considerable detail.