I am not altius, but I think he and I agree about this particular aspect of technique. I feel that this (Greg D.'s first jump) is exactly the type of flexion that is desired to accelerate the swing. The flexion increases the "snap" and the leg still locks out to its maximum length through the chord of the pole, which is where it matters! Those who try to keep it long without the flexion end up having to tuck because their swing is too slow. I believe that most of the guys you listed actually do snap the knee into the swing to varying degrees...Tarasov the least.
The hips still stay back just fine with this degree of flexion, and Tim Mack demonstrated that an incredibly fast swing can be generated with a relatively large flexion at the knee. Try it on your rope/rings/highbar swing drills, and on your small poles/short run, and see whether it helps you get the swing moving faster or not?
We can learn a great deal from watching a golf swing, as to why a flexion at the knee, can have positive effects on the swing. Ben "The Hawk" Hogan had one of the smoothest swings of all time, and still hit the ball pretty far and straight (and could shape the flight of the ball as well):
The hinge in his wrist, is what gives great clubhead speed at the the moment where the face hits the ball. Try locking the wrist (gripping to tight on the club severely impacts clubhead speed due to the fact, that it tightens up the wrist), and swing without hinging. The result is loss of clubhead speed.
Now, in response to Mack's trail leg: His flexion at the knee should increase the speed of his swing at the bottom of the arc (when he hits the cord of the pole), which should in fact compress the pole (the advantage of a powerful intentional swing), and add lots of energy. His tuck is definately not a result of an inferior swing. Now, the reason why I wouldn't advocate hinging this much is, that I has been taught to hit the pole with a solid body at take-off. This include the lower extremities. IMHO, at take-off, should the mass of the entire body be pushing the pole upwards and inwards. "Letting go" of any part of your body after take-off, lets energy bleed out, and the chances are, that you cannot recover fully from this loss. Mack's flexion IMHO lets him recover some of this upward and inward momentum, and utilize it in his swing (but I think there's still a loss in this conversion).