USA is not the most diverse country (its ethnic fractionalization index is below that of Moldova - https://worldpopulationreview.com/country-rankings/most-racially-diverse-countries or https://en.m.wikipedia.org/wiki/List_of_countries_ranked_by_ethnic_and_cultural_diversity_level

Idk, both of those studies leave a lot to be desired. The first methodology is based on language, which ignores the history of conflict and forms of imperialism that used language as a tool of imperial expansion.

The second study doesn't count any culture that doesn't make up more than 1% of the total population. Which means countries like Papua New Guinea, one of the most ethnic diverse countries in the world, doesn't even register as they don't have a dominant ethnic group.