Yes, but what if that horse put on the brakes and stepped back. He could easily dislocate his tail. No matter how desensitized, that is something I would never do.
It looks to me like the tarp was tied on to tail hairs below the bony part of the tail, so if there was a sharp pull on it the hairs would break first (assuming the tarp stayed attached). In any case, where does one draw the line with risky horse activities? Jumping cross-country, what if the horse/I misjudged the obstacle and he somersaulted over it. He could easily sustain serious injury. That is something I would never do (really!).
Yes, I did enjoy it, I think she was an excellent trainer, her horses clearly love her, and I would kill for her (riding) seat. Remember, I said tying the tarp to the tail is something "I" would not do. I think I could demonstrate the same thing without risking a tailbone injury to my horse. But you are right, most activities we ask a horse to do have some risk of danger. I just have a different perspective about the risk/reward aspect of the tail tarp.