Did Google just defeat every geofence warrant?

Not really. It makes it harder, since the warrant would now have to include the cell-provider(s) for your mobile device(s), which would all them to geofence you by tower location or the HTTP headers (which Google could also be compelled to provide). And they would need to correlate the various data to be able to get a rough idea of where you were, as opposed to getting that data in one step from Google.