Malthus, in looking at the conditions of North American natives during the 19th century, reports on the dire conditions of a people devastated by introduced diseases, direct conquest by white settlers, and the disruption of their social fabric and ways of life.
Some of the accounts presented by Malthus were given by very early explorers and adventurers who ended up deep in unexplored territory, far ahead of European conquest and colonization. For example, the one by Cabeca de Vaca would be circa 1530.
The only way these societies could have already been devastated is if epidemics had ravaged the whole continent immediately in the first decades after the first Europeans landed, ahead of any European contact with the inland peoples. I don't know enough about the relevant history to know how plausible this is, but even if it happened, there are two problems with your claim:
Diseases wouldn't cause famine, at least in the long run. These early explorers describe peoples who had problems making ends meet during bad seasons due to insufficient food, and who fought bitterly over the existing limited supply. If the population had already been thinned down by disease by the time they came, we'd expect, if anything, the per capita food supply from foraging to be greater than before.
If even the earliest accounts are of devastated societies, then how do we know anything about the better life they led before that? Where does this information come from? You cite an ethnography by Boas, who was born in 1858, as authoritative, but dismiss a compilation of far older accounts compiled by Malthus in the early 19th century.
Smallpox emerged in the Old World around 10,000 BC and is believed to have originated via cattle farming. It reached very high concentrations in Europe and became a common plague there; it was spread around the world to peoples who had never encountered it by European exploration and conquest. It and other Old World disease spread very rapidly among American native populations, rendering whole cultures extinct and reducing others to scattered survivors often incapable of rebuilding. The total population of the Americas lost to European diseases after the a...
From Mike Darwn's Chronopause, an essay titled "Would You Like Another Plate of This?", discussing people's attitudes to life:
Conclusion, graphs, and references in article. As usual, I recommend reading Chronopause.com as Darwin has many good articles; to quickly link a few: