> they couldn't have been more than a decade behind the smallpox/diseases
I think it was much more than this. Remember Columbus landed in 1492 and these areas were very connected with trade networks, including from the Carribean islands to the mainlands. Diseases could have spread through most of both of the continents in a decade. And then places like what became the United States were not regularly seeing Europeans until ~1620, so the apocalyptic plague would have already had a 100 year head start. Places like the West Coast and interiors of the US or interior of South America would have been 200 years or more post-plague.
Also, many of the earliest explorers of the deep interior often described sophisticated and heavily populated societies. But when others eventually reached those areas to find them sparsely populated wilderness those stories were dismissed as just more legends and rumors of cities of gold. See Francisco de Orellana's description of his expedition down the Amazon for a good example of this.
I think it was much more than this. Remember Columbus landed in 1492 and these areas were very connected with trade networks, including from the Carribean islands to the mainlands. Diseases could have spread through most of both of the continents in a decade. And then places like what became the United States were not regularly seeing Europeans until ~1620, so the apocalyptic plague would have already had a 100 year head start. Places like the West Coast and interiors of the US or interior of South America would have been 200 years or more post-plague.
Also, many of the earliest explorers of the deep interior often described sophisticated and heavily populated societies. But when others eventually reached those areas to find them sparsely populated wilderness those stories were dismissed as just more legends and rumors of cities of gold. See Francisco de Orellana's description of his expedition down the Amazon for a good example of this.