No, it's one small item of 'determining factors'.
Yes, the richest zip codes are in cities, but most impoverished areas in the USA are actually generally in cities. The former drags the average way up.
Have a look here, Postal Code map of Canada [1]
Scroll over to to Toronto:
You notice 'super rich area' uptown. Then downtown and environs and immediate burbs are poor, some very poor. Then the 'extraburbs' are wealthy, then then countryside in the middle.
If you account for cost of living - the countryside people are doing very well. It costs $120K for a nice big house in my hometown, and electricians earn $80K.
In the us - the rural areas in the US that don't have a history of slavery (like the Dakotas, even Texas) actually have high levels of income and low levels of unemployment, low levels of crime ... but as you point out, not quite the same as the 'professional class' within cities.
So the 'banker class' and 'professional class' will be in cities, and make the most money.
But many of the rest in economic zones actually don't fare very well.
[1] https://globalnews.ca/news/370804/income-by-postal-code/
In modern times, we understand that access to markets is the determining factor in income. It is why those who live in cities can typically make more than those who live in rural areas, and why – as you alluded to – those with family connections to business find themselves making more than those who don't have an "in." It is even why people strongly advocate for limiting competition (through unions, for example) in a marketplace in order to increase incomes (or other benefits) of those who are in the inner circle.
One would naturally conclude that missions were correlated with access to markets, perhaps due to springing up where markets were already strong, but the article seems to have determined that education caused the higher incomes back then. Something that hasn't proven true in modern times, where incomes have held stagnant over the years for groups that have seen an increase in college attainment.