This article is a direct criticism of the dominant ideology of the United States and I explicitly say that our interventionism overseas has led to tragic consequences. If anything my thought is based on the experience of global indigenous communities like Rojava and the Sandinistas.
Singapore has authoritarian tendencies but it's still a parliamentary republic. I don't define diversity as just the presence of different ethnic groups, but also the toleration of those groups. This is a complicated subject but ultimately an authoritarian mindset tends to label cultural diversity as a threat to its dominant position. This is more complicated than just the label you put on the form of government: differences in class, race, ethnicity, and so on lead to marginalization of certain groups.
As a finer point, I don't believe that different cultures exist because of separation. The idea of nation states is very recent and for most of history you have gradations of culture that differ slightly from each other over distance/time/etc. A good example of this is the romance languages: Spanish, French, Portuguese, Italian, and Romansch are all descendants of Latin dialects that originally existed along a continuum before the imposition of modern nation states with standardized languages. Again, the reason why diversity was eliminated in these regions was because of centralization and forms of domination which I believe we must find alternatives for.
I tend to be optimistic about human beings, but ultimately there has to be some sort of agreed upon morality for us to even begin discussing if people are fundamentally "good" or "bad". My position is humans exhibit a wide range of behaviors and I personally judge behaviors that improve the well being of other consciousnesses to be good. I believe we need to work on building systems that promote human flourishing.