Turning off the transponder doesn't make the airplane invisible to radar (unless it's already a long, long way off), it just makes it unidentified. There's still a primary return.
1. Unless they’re deliberately flying low or otherwise to avoid such return, and 2. I have no direct knowledge of the circumstances, but, this is what was reported.
I fly a lot, you can easily avoid ATC radar by turning off your transponder and heading for the deck getting under the radar horizon which for a 50 foot tall tower is only 8 miles out. The FAA will pull your very for such shenanigans but physics is on your side.
Remember a Cessna in the 1980s turned off it’s transponder in Eastern Europe then flew through the most heavily defended airspace on earth past dozens of military radars , SAM missile defense systems and landed wheels down in Red Fing square in the heart of Moscow. So yeah turning off a transponder and getting to the deck you vanish. You would need airborne AWACS with military grade skin return only Doppler look down radar and even then in the clutter a small aircraft can hide, follow a roadway and stay at or just above freeway speeds the moving target indicators will ignore “road” traffic.