Ask HN: Can you use geofencing (e.g. Foursquare) for contact tracing?

Apple and Google are going to use bluetooth to track person-to-person contacts (https://news.ycombinator.com/item?id=22834959), but Foursquare, FB, and Google already have geofencing to track when users go to specific locations and for how long. That doesn't help with subways and buses, but wouldn't it still make contact tracing a lot easier, particularly in the suburbs?

I just glanced at the Foursquare docs (https://developer.foursquare.com/docs/pilgrim-sdk/), but I feel like a quick React Native app could be built before the Apple/Google May API release date. And that a dev would spend less time worrying about privacy and crypto because it's existing tech and users have already opted in.

Any thoughts? Thanks!