Yeah, but it’s easier to catch murderers at a nearby residence with additional physical surveillance than it is to prove ballot stuffing in a public location in a residential area as the GPS can be off by several meters.
Supposedly, the film assumes any cellphone that had additional pings by a church, library, police station, post office, or any other public areas used as temporary voting locations. Any location like that that had more than 10 pings by a single cellphone was assumed to be a mule.
Naturally, there may be other reasons such things may have happened besides the one alleged in the film.