Agreed. The most desirable and expensive neighborhoods tend to be the most walkable ones. It would make sense that the people who live in those neighborhoods tend to be more financially successful.
It's as if a study came out that showed that, according to their exhaustive statistical analysis, people that drove Mercedes tended to be wealthier than people who drove Chevys. Then they concluded that the key to upward mobility must be to get a Mercedes.
It's supposed to be mobility, and I think tracking individuals through their tax returns. (This magic data which only Chetty & co have access to.) So I don't think the Mercedes example works.
But I agree that it's very unlikely to be that being able to walk places makes you succeed. Could even be something as simple as enough people owning a few flats in their neighbourhood, which suddenly became super-valuable when lots of google employees showed up. That would look a lot like income mobility.
It's as if a study came out that showed that, according to their exhaustive statistical analysis, people that drove Mercedes tended to be wealthier than people who drove Chevys. Then they concluded that the key to upward mobility must be to get a Mercedes.