In speaking about city dwellers, I wasn't thinking about suburbs, exurbs, and gated bastions. People who live there are possibly even more isolated than the other two groups. Places like Fifth Avenue and such are off my radar screen. Those people tend to be older, and I'm really not tuned in to their homogeneity or lack thereof.
I wasn't speaking of suburbs, etc., either, unless you're referring to my mention of Santa Monica. I'll admit, I don't really understand how Los Angeles works. The other diverse pairings I cited are all in cities. I don't have much in common with Fifth Avenue denizens either, but that's exactly my point -- that city is extremely diverse -- or at least was during the brief period I lived there; I'm sure rising housing costs are eroded some of the diversity. (Although what I mentioned were residents of the Upper West Side, who probably have a fair amount in common with Denver residents, though probably higher median income). Same for other cities with rich and poor neighborhoods, including my own.
But here in Denver we just had more than 20,000 people at our annual MLK "Marade" which is both a march and a parade. We also have big celebrations for the Day of the Dead and Cinco de Mayo. Our governor is gay. Many neighborhoods are becoming more diverse or being gentrified, whichever way you want to look at it. You can easily get gender-affirming treatment or an abortion, for example. More and more places are bi- or trilingual. Our performing arts center, which has won an Emmy for best regional arts, has been a champion of Black playwrights and BIPOC actors and dancers, and the audiences have grown more diverse.
But wait, weren't you arguing that cities are less diverse/more homogenous? I'm not sure of your point here. You mentioned a lot of different marginalized communities, although you said you weren't talking about them, either. Sounds like what you're describing is a nice, progressive, 21st century city.
If we're talking strictly politics, it's possible that people in cities (as you said, not suburbs) are more homogenously blue-voting, whereas although rural people may tend to vote red, it's probably not as universal because there certainly are Democrats/lefties/progressives in rural areas and small towns.
The local media have done series on the problems in rural areas and many of the institutions that brought people together are dying out. Many people, particularly older ones, are isolated and younger people are leaving some of the rural communities.
Yes, as they have for decades if not forever, leaving the rural dwellers more homogenous, not less -- more similar in age, background, race (if we're talking about that), etc.