> But I don't think you should go around telling people that a keto diet will cure them of their psychosis

They didn't say this. They described their own experience. Read their message again. You are, in fact, criticising the post for something not written in it.

> People should make their medical decisions based on consultation with their doctors or other medical professionals

This is exactly what the user described as their experience.

If you, god forbid, ever have a life-destroying ailment and discover a shockingly effective treatment for it, please tell many people what happened for you just as the user above is doing.