I don't have a replay on hand so I can't say whether I agree with him driving up or not, but your physics statement shows a lack of understanding of fundamental concepts. Basically, it is entirely incorrect.
If I am running and I jump, and then my lower half hits a stationary object, then my lower half will stop moving and my upper half will continue moving. This will cause my upper body to flip over the lower half.
You can do a simple experiment for yourself if you don't believe me. Grab a pen. Throw it upright into the side of your hand, so the lower half (about 1/4 up) hits your hand, and watch it flip.
Now, drive your hand up, or down, and it still flips regardless. Once someone jumps into an already low tackle and they get hit around the knees, they are pretty much always going to flip.
I don't have a video for the Greene tackle so can't confirm how low he was, but with this in mind, the responsibility shifts to the ball carrier. If the ball carrier jumps into a tackle, then the tackler can't be held responsible if an otherwide fine tackle turns bad.