They're not completely wrong. It's just not Flock.

Police have had Stingrays for well over a decade now, which are basically man-in-the-middle attacks. They mimic a real cell tower, so your phone connects to it like a normal cell tower. Then the police have full access to all transmissions your phone makes to that tower. That doesn't give them access to things like your contacts that are stored encrypted locally, but it does give them access to your text messages in transit (if not E2EE encrypted), your voice data during transmission (so any phone calls you make they can listen in and record), and any other metadata in transmission.