Ok, I had a brilliant lengthly reply drafted and fat-fingered it away. The short reply.
No they clueless.
The sun is not wired for Alternating Current or Direct Current. We don't have extension cords that long yet.
No doubt it's micrwave emissions vary with time, but that doesn't make them 'pulsed DC'.
A microwave oven works by transmitting a signal at the resonant frequency of the water atom. The atoms vibrate (I hesitate to agree to 'friction" since I think atoms seldom actually touch). The vibration transfers energy to adjacent atoms "heating them". Essentially, you are pumping energy from the electric mains via a radio trasnmitter into the water atoms and into the rest of the food.
I can't see anything dangerous about this. Commercial ovens often use infrared light (heat) to cook food. Home ovens use infrared and convection. Just a different frequency of electromagnetic energy. Campfire is the same.
What do they want people to do? Stand outside and hold a roast up to the sun for natural microwaving?