My first reaction was that the frame rate of the camera happened to be synchronized with the speed of the prop, but it looks like the chopper rises at one point, meaning the props wouldn't be at a consistent speed, so you'd see motion. So I really don't know.
What supports this is the illusion that the tail prop is spinning extremely slowly, again because of the camera framerate.
There's a video I saw a long time ago of someone doing a power slide in a Porsche, and the framerate made the wheels look as though they weren't even spinning. You see this kind of illusion in videos all the time.