Both aircraft have to be equipped with the system for it to work. It's common in airliners, perhaps not so common in military helicopters.
They both had transponders on in this case. It would be a suicide to have a helicopter base close to a major airport without mandatory helicopter transponders.
It seems to be a disaster caused by miscommunication/misunderstanding. According to one of the IMO best aviation channels on YT ('Blancolirio', if anyone's interested), the regional plane was flying a perfect approach to the runway (with a challenging sidestep maneuver included, but the plane performed it correctly). ATC cleared the helicopter to pass behind the aircraft. For some reason, the helicopter crew agreed to do so, but then flew right into the plane from the side. Those are all verifiable facts.
The channel author then offers a fairly plausible explanation, saying that the helicopter crew might have mistaken the jet they were supposed to be looking for for an aircraft taking off further away, and approaching from the side, they might not have seen the bright landing lights of the landing aircraft which are only clearly visible from the front.
Also, he says that below 1000 feet, TCAS just says 'traffic' without any further instructions to descend or climb as the aircraft have no room to descend. This happened at less than 400 feet.
Still praying for a miracle
[edited so that the reported speculation wording doesn't ruffle feathers]