I find it insane that the fact that Google is constantly harvesting device locations is blatantly obvious from things like this, but yet if you make that claim in other contexts people will be extremely skeptical and demand extensive proof.
Those phones all had Google Maps app open. That Google is harvesting device locations while you have a Google app open showing your GPS location should not be a surprise.
What would be a surprise is if Google forced on your GPS to gather your location in other contexts.
It is not a surprise, it is already happening. You can turn it off, but AFAIK if you don't it collects this information even if the Google maps application is not open.
From my personal experience, I have an Iphone and it asks me about once a week about Google Maps using my location in the background and if that is okay. I always tell it no. I didn't know I could turn it off directly in the app, thank you for the information.
GPS is definitely the best way of ascertaining a device's location but far from the only one. Google's own location API lets you access fine location (GPS) or coarse (triangulating via cell towers).
After public outcry and legal action Google stopped driveby data harvesting by Street View vehicles but it remains unclear if they stopped collecting SSIDs and MAC addresses (I guess if you squint hard enough you could say that these are publicly available data points).
It doesn't take too much tin foil to think that android may be phoning home when it detects an SSID, the physical location of which is already known.
Google absolutely harvests phone GPS data to enhance things like AGPS (mapping of wifi MAC addresses, or celll towers to location coordinates), and it does this regardless of the location setting in the system UI.
And Apple too - I am constantly seeing this (traffic/congestion flags) on side and residential streets with no traffic monitoring devices in my Apple Maps.