Suburbs are typically more socially homogenous, with more institutional connections between residents (kids go to the same school, people work for the same local employer, same church, etc) with a physical environment less conducive to connectivity. City neighborhoods (again, typically) have better physical presence with neighbors that are less likely to have things in common. I think that's what the author is trying to say.
An old rationalization of prejudice. Everyone seems homogenous to me and what was heterogeneous yesterday (e.g., Italians and Irish) is homogenous today. Just stop worrying about it. People with different backgrounds are much more interesting, all else being equal. All are Homo sapiens.
Also, kids in city neighborhoods also go to the same schools. In suburbs I've seen people don't generally share an employer and church - that's a small town.
It depends on the definition of suburb (some are pretty urban), but my experience in cul-de-sacs is neighbors rarely interact. Lots of places don't even have sidewalks.
Those were generalizations covering 100M+ people in each category, just in America, and my guess at what the original author meant by the statement that GP was surprised by.
