There is almost a perfect inverse relationship between how modernized a country is and where it appears on this list (with obvious exceptions for repressive dictatorships like North Korea which aren't modern, but because they're repressive dictatorships with closed borders and ethnic cleansing, it skews the trend).
The list in this order seems to confirm that as countries become more modernized, diversity is sacrificed for physical and social mobility. I think it's considering diversity as something very different than the typical Western ideal of diversity, and in that way, the diversity that this represents is very harmful for a more equal society. Western liberal democracies typically strive for classical diversity as an ideal, but it would seem that the method here suggests that once a country moves towards being more liberal (classical liberalism here), more educated, with easier social and physical mobility throughout the entirety of the country, that diversity is sacrificed.
Russia is a good example of a highly diverse, but blatantly unequal society from this list. Western Russia is a modern country which probably has a fairly homogenous society (or at least, on par with Europe or the Americas), but outside of Western Russia, the rest of the country has relatively little social or physical mobility, desperate conditions, and very little connection to the Russian government. Should the entirety of Russia become more modernized over the next 30 years, I'd imagine you'd see their rank on this list drop, as the ability for people to move seems to make a country
less diverse, which should almost be a paradox if we're thinking of the classical ideal of diversity. Consider that if the United States in 1859 were on this list, it would perform better than the United States of today, because approximately 15% of the country's population -- indentured slaves -- were barred from travel, barred from learning how to read, barred from sharing other cultures (and becoming more homogenized), yet this would have led to more distinct diversity. A good portion of African Americans in 2017 are descendants of slaves, but because of social and physical mobility, the methods of this study would (rightly) consider them less diverse today than their ancestors 200 years ago, who were withheld the right of education and mobility which, as the method would suggest, preserved their diversity.
So, given that, I think I'd be a little hesitant to celebrate being higher ranked on this list.
Probably because it's not necessarily reflected in the culture or media.
I'd imagine Canada's ethnic diversity in this list is due to its strong native population with distinct cultures, where as the native American culture in the United States has been roughly assimilated (historically through force, coercion, today largely through proximity). Canada and the US are in the same ballpark, so I think those distinct native populations bolster Canada's rank.
Though, I'm not really sure if you want to go much higher than 50 in this list... Of the top 30, an overwhelming majority are the most illiberal, autocratic, unequal dictatorships in the world. It makes sense that the method of this study would consider them more diverse, because lack of social mobility, lack of technology, lack of country-wide education, and rampant disease, squalor, and malnutrition keeps nationalities distinct within these countries and leads to very little homogenization, mixing of cultures, or reproduction between distinct groups.