I banged my head on this four many hours and couldn’t get it to work directly in Linux. The key issue seemed to be that ADE and Kobo had to be installed on the same device, so I’ve resorted to using a Windows VM (VirtualBox). It works fine enough.
Most iPhone purchasers at that time were high income, as it was an expensive new technology and the general consumer was happy getting ‘free’ phones from carriers. It was also costly and difficult to switch carriers. And further, I know from having worked in banking that it is very difficult to get accurate income data.
Taken together, my guess is this is just detecting the effect of higher income on lower birth rates.
When nobody pays taxes, there’s plenty of dry powder to prop up bad economics.