There were MLAT tracks for the helicopter, which is where the flight path information everyone has been using came from. Also is responsible for the odd discontinuities in the flight path data. MLAT triangulates Mode S, ergo Mode S was on. Here’s corroboration from a USA Today article, though:
“However, the helicopter was not broadcasting ADS-B and was broadcasting only Mode S, a secondary surveillance and communication system.
ADSBexchange was able to triangulate the flight path of the helicopter using multilateration (MLAT). was able to triangulate the flight path of the helicopter using multilateration (MLAT).”