If they were at the same height on a collision course then to each other their lights would appear stationary on or near the horizon. Its possible there were some other lights that the helicopter pilot mistook as the plane. From the crash video it appears there was another plane taking off and it's possible the helicopter pilot thought this was the traffic they had to avoid. Strobe would be the only clue the lights belong to air traffic but would be hard for the helicopter pilot to see drowned out by the planes landing lights. Maybe they will fly a reenactment to find out what each pilot could see.

PS reportedly the helicopter crew were wearing night vision goggles that might restrict their peripheral vision or make it harder to distinguish really bright landing lights on the plane from normal bright lights.