All these blanket assertions are, of course, wrong, but this case borders on the absurd. There were standard violations galore in the Tancredi case. A professional has the responsibility to provide care within reasonable standards, and when those reasonable standards are violated, then the professional is indeed at risk of being held accountable when something goes wrong. This dive was far outside of standards.
This is stressed in instructor training. If you do not stay within accepted standards, you are at risk and may have to demonstrate why the non-standard practice you followed was acceptable. If you stay within standards, then you do not have to do that.
I therefore so not see why the Tancredi case is relevant, unless you can show that this instructor was similarly in violation of standards.
Tancredi is as far exaggerated to one extreme as the idea of an instructor being responsible for a diver struck by a meteorite in the water is to the other. I think we can all accept that there are situations where a reasonable person would find an instructor to be liable for a divers injuries just as there are situations where one would find the instructor free of responsibility. Let's not argue the Tancredi case here...