I believe a GSM tower has about an 8 mile or so radius before it switches, so even if it were just tracking tower connectivity it could be fairly accurate.

And if you switch at the edge of one tower to another tower, that would locate your phone pretty precisely. I’m also assuming that if you have a dropped call / disconnect, the time it takes for that to happen could pretty well locate where you are in relation to the distance from the tower.

I think this article slightly understates what this is able to do.