There’s some more recent scholarship than Guns, Germs, and Steel. See Rationalizing Epidemics: Meanings and Uses of American Indian Mortality since 1600 [1]. The truth is maybe a bit more complicated. We had doctors volunteering to visit tribes who recorded what they observed firsthand.
Personally, given the evidence at hand, I think it’s likely the populations on this continent were caught in large boom/bust cycles, and we happened upon them right at a bust cycle. It’s definitely up for debate. There’s also modern work on smallpox using genetic clocks etc to consider.