I don't think that video disproves the spiraling slip stream theory. Are you expecting to see the smoke spiral like a cork screw? I've been flying professionally for 20 years; everything from a C152 to various turboprops to heavy MD-11s. I also instructed for several thousand hours in single/twin recips and turboprops. Many books and training manuals refer spiraling slip stream. I have a hard time believing they are all wrong. One video with no "obvious" spiral of smoke doesn't debunk the theory any more than than the video proves there is no gravity because the airplane flies away from the ground!