That seems like a fully general counterargument against relying on medical diagnoses for anything. There are always facts that confirm a diagnosis, and then the diagnosis itself. Presumably, it is often helpful to argue that the facts confirm the diagnosis instead of simply listing the facts alone. I don’t see any principled reason for eschewing diagnoses when they are being used to support the conclusion that someone’s testimony or arguments should be distrusted.
That seems like a fully general counterargument against relying on medical diagnoses for anything. There are always facts that confirm a diagnosis, and then the diagnosis itself. Presumably, it is often helpful to argue that the facts confirm the diagnosis instead of simply listing the facts alone. I don’t see any principled reason for eschewing diagnoses when they are being used to support the conclusion that someone’s testimony or arguments should be distrusted.