I just listened to the ATC recording from immediately before the collision. ATC instructs the helicopter to pass behind the CRJ. I’m fairly certain a few minutes before that, ATC instructed the helicopter to maintain visual separation, which is common. They typically ask, “do you have the aircraft in sight” and if you respond in the affirmative they rely on you to maintain safe distance. I should mention that in the…
In the video from the webcam there's another plane which is much easier to see. Could they have asked about "the aircraft" and the helicopter pilot mistook which one they referred to? "yes I can see the plane flying much higher"
That said, it's possible they mistook which aircraft to look for, but it's unlikely imho and we will likely never know for sure, as I would presume the pilots are deceased.