More is possible than you might think
The basic idea of WIFI targeting is serving ads on phones, where the location is determined by a WIFI point. This is indeed possible. There is more to it, however.
It’s technically possible to upload/insert IP addresses of - let’s say – branches of a certain brand/retailer. Then, when a consumer – physically visiting one of these branches – access the internet using this free WIFI point, ads can be served. Furthermore; retargeting activities, based on these impressions, are possible. Logically the storage of (RAW) Device-ID’s is also a possibility.