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.