That is complete bull****. Microwave radiation is not stored anywhere in cooked food, the waves simply move the water molecules up and down as they follow the sinusoidal changing electric field so that they warm up through friction. The food cannot become a source of microwaves itself.
It is obvious that the author has no idea of what physics and chemistry are.
The "brain de-magnetisation" thing is plain ridiculous and needs no further address.
Equally preposterous is the claim that microwave radiation could cause significant DNA damage: this kind of electromagnetic radiation has a much longer wavelength and lower energy than that required to do that. That is the kind of problems associated with ultraviolet (only as far as the skin is concerned and after prolonged exposure to sunlight), x- and gamma rays, which are several orders of magnitude away. Completely out of the question.
"Minerals, vitamins, and nutrients of all microwaved food is reduced"
This is the only statement which is likely to be true in the whole text, yet it is unlikely to be in a direct relationship with microwave. Many organic molecules are heat-sensitive, and they would decay under any cooking process.