Anybody can sue you, whether it's justified or not. The question more importantly is whether you can defend the suit. In my opinion, if you identify the indicators you have seen, and define the logic behind your decision, even if your analysis is wrong, then your accountability if minimized. After all, the only way to be 100% certain that a tree doesn't have defects that could make it dangerous would be to do a complete destructive sample. Every field, such as engineering, makes assumptions based on observation, experience and experimentation.
There is a big difference between doing a good job of identifying indicators and lying your ash off to get a sale. Playing a game of "cover your ash to avoid loosing in court" won't help much if you are still a lying fraud out to rob the customer.
Clearly, the judge figured they done wrong, and he had all the available facts.
I don't ever worry about lawsuits. I just do the right thing, tell people the facts that I know, those that I don't know, and I never sell any tree work just to make money. I sell good service and expertise to make money, and sometimes that entitles me to collect for tree work performed.