The one patient in this story who got a prompt and serious investigation was found to not be delusional. Was this because anything about his story was more credible? No, it was because he was his own doctor, and strangely not satisfied with deceiving himself. If any other person with the magic doctor bit has had to deal with the problem personally, we aren't told.
This story depicts desperate people who have probably-wrong ideas about their condition, and a medical profession with probably-oversimplified overconfident ideas about it which it has no interest in improving -- they're the opposite of desperate, and so much the less excusable. Put yourself in the shoes of this guy, with his doctor bit erased: if he tries to get help, and explains his whole problem, he knows he'll get classed as delusional, his doctors won't even try to figure the thing out, they'll say "You're very anxious" and get insistent about anti-anxiety drugs, NSAIDs, and seeing a psychiatrist. (If he's lucky like someone I know, after a few years of persistence and ignoring the drug prescriptions he'll find someone who can diagnose an underlying cause, which in that case -- not about itching -- was not mites but gut flora, manageable with a diet change.) He should worry about deliberately mislabeled drugs too? I hope my question was over-cynical.
This could be read more charitably, but this sure sounds like deceiving the patient. Is that what they mean?