OK, I am not an attorney, and I know some attorneys are involved in this thread. I am going to make a layperson's understanding of the potential impact of IANTD's new standard on the case and invite more learned comment.
The existence or lack of existence of standards can have a major impact on a case. If I am instructor who carefully follows standards and am faced with a suit following an accident, the fact that everything I did was within standards puts major burden on the plaintiff to prove that what I did was unsafe and at least partially responsible for the accident. Because my actions were within standards, then there is a strong implication that everything I did was reasonable and within the best practices of scuba professionals.
If I violate standards, then the burden will be on me to prove that what I did was safe and within the best practices of scuba professionals. That will not be easy for me.
If there is no standard for what I did, then the plaintiff will try to provide evidence that what I did was unsafe, and I will have to respond with evidence that is is within the best practices of scuba professionals. What evidence might a plaintiff bring in the absence of standards? The near consensus in the ScubaBoard thread that 3 dives beyond 200 was too much would give them a wealth of possibilities. Another piece of evidence that they might bring up is the fact that IANTD belatedly created this standard, which indicates that they do not think 3 such dives are safe. I would have to respond in some way to all that evidence indicating that other professionals did not think it was safe.