Basically this research combines a legacy IPv6 addressing scheme where the MAC address is put into the address with crowd-sourced WiFi network scanning geo-location databases. The trick is figuring out the delta in MACs between the WAN and WLAN adaptors, but they are usually close, so with some clustering, they were able to get 39m accuracy for ~12M routers in over 100 countries.

Crazy to think putting a MAC address in a world-routing IP address was ever considered a good idea, but with networking gears' long life cycle, it will be a long-lasting mistake!