Of course I was responding to you, I quoted you responding to me. I stated that the doctor was the one who decides medical fitness and you replied to that with "an instructor signs off", so I was trying to understand what you were trying to say. It seems, based on this response, that you agree that the doctor and the patient/student decide fitness to dive, and the instructor decides if they're willing to teach or not.
I happen to believe that any instructor unwilling to teach someone because they don't have the medical knowledge to understand that the person is fit to dive, despite the student and their doctor saying they do, has a personal issue that is screwing up their ability to do their job. If they think a person lied etc about their medical conditions, that would be a different scenario; but for those who get their doctor's approval for diving I can't imagine a good reason any competent instructor would override the decision about their medical fitness to dive that a qualified professional doctor has made. Others may not agree with that assessment, but I've seen no good reason to think that isn't the most obvious conclusion to come up with for such people's decisions.
We agree on most things. I was referring to the scenario of an instructor with a dubious med condition. If they black out at 30’ with six students, it’s a much more serious event than a student doing so.