There's tracking a phone while it's off, and then there's realtime tracking of a phone while it's off. Two very different things, and two very different attacks.

"a new NSA technique enabled the agency to find cellphones even when they were turned off"

The current administration is very careful with choosing their words. I haven't seen WP's source, but I wonder if this is more about the phone blipping its receivers to record some local MAC addresses and scrambling codes and then uploading the data the next time the phone's powered on.

You know when the word "collect" doesn't mean what you think it does, I wouldn't bet on nailing the word "find". :-)