APY it is not defamation if it is true even they were damaged by the remarks.
For example if I say John is a molester, he sues me becasue now his child care he runs in his basement is suffering. If it turns out he is a molester, I am not liable even if his business suffers. In order to determine liabilities the court will decide if my remarks were true.
Now on to kashrus
If I say Shirley’s organization is unrelaible and her “hechsher” business suffers. In order to be found liable the court would have to determine if I am right or not. However, as you correctly pointed out, “The law will not decide whose standard of kashrus is correct ” Therfore the case will get thrown out even if her business suffers since the court can not determine whose interpretation of Kashrus is correct.
This isnt limited to kashrus. If I say a mikva isnt kosher would be the same thing
Any specefic (recent) case you can cite that says anything different?
(In the old days there were laws protecting Kosher. Kosher was determined to be only by orthodox standards. Then (not anymore in NY) the opposite of what you say could occur, since I could ahve sued Shirley for claiming to be Kosher when she wasn not by orthodox standards. But this is moot today)