I can't imagine DB not being an insider. He/she had to have known a lot about Boeing 727s to pull that off. There are panels both inside and outside the aircraft to operate the rear stairs, and when the stairs were used it was usually operated from the outside panel. An inside operator had no way of knowing it was safe to lower the stairs. A non-insider would likely not even know if there was a panel on the inside. Also the controls themselves varied a bit from one plane to another.
Also, there was no way of knowing how fast the plane was going for a rear jump. And too fast is dangerous if the chute is opened too soon. He also didn't know the altitude, and could have drifter 50 miles before hitting the ground.
BUT ... an insider would also have had to also be a fool to try this with so many unknowns; and I don't remember any pilots, flight attendants or maintenance personnel who I'd classify as fools,
