Could also have to do with the city's wealth itself. A city that has money to spend on trees, probably has better infrastructure, more jobs available and and just more ways for lower income people to live and thrive than a city that can't afford to plant and maintain trees. Trees are expensive. It means they probably invest more in stuff like accessible transit, maintaining roads and just other general quality of life improvements in the city.
I figure trees are probably lower down on the list of things to spend money on, so if they've got the money for that, the city itself is like just a better place to live for a ton of other reasons.
I expect wealthier people tend to live in nicer neighbourhoods that just tend to have more planting, those people are more likely to have better lives than poor people who happen to live in neighborhoods that can't afford to plant a lot of trees.
In saying that, I'm still all for planting as many trees in cities as we can. This just seems like a flawed study