Diversity and segregation are two different things. If you had a city that was 100.0% Vietnamese then there would be no "segregation" within that city but it also isn't particularly diverse. And adding an equal number of Hawaiians wouldn't reduce the diversity even if there was 100% segregation between them.

Moreover, if you can bisect Asians into Japanese and Koreans etc. then you can bisect whites into Irish and German, Latinos into Colombians and Venezuelans, blacks into Jamaicans and Haitians etc. And if you're going to play that game then why aren't the racially segregated areas still internally diverse in the same way? Certainly they are as much as the countries that consist almost entirely of that category of people are.