The venomous diatribes against real medicine, and science and rationality in general, behind the closed doors of the "alternative" movement should tell you all you need to know about the sincerity of the quack-my-ass clause. On the face of it, it sounds obvious, egalitiarian and big-hearted. However, the real intention of the advice is to ensure that if anyone dies or is incapacitated by taking the quack's advice or products (or by swearing off real medicine, which may not be advised on the packaging but is a stock in trade in the "alternative" industry), if the matter comes to court the quack's lawyer can claim that the product or the service was misused; obviously, they didn't check with their GP, how unfortunate, it's not our fault.
Besides, the quack knows perfectly well their client is unlikely to see their GP or specialist about whatever the problem is, or if they do they won't pay much attention to their advice. If they did, they wouldn't be coming to the quack in the first place.