All a stillsuit was supposed to do was preserve as much moisture from escaping as it could. It was not anywhere near 100% efficient even under the best conditions. It required the wearer to breathe in thru the nose and out thru the mouth. Urine and fecal matter was collected in side leg pockets and moisture recycled. Even with a good Fremen-made stillsuit, a person could only last about two weeks in the open desert of Dune with no replenishment of water supply........................
This is a physical impossibility to survive without exterior energy sources or significant water.
The temperature of the air is given at about 72 degrees minus 25 degrees Celcius. That is 48 degrees C or 118 degrees F. When the air temperature is above 100 degrees F, the body cannot cool itself by conduction. It has to cool itself by radiation or evaporation of water.
With the ground temperature much higher than the air temperature, cooling by radiation is essentially nil. It is far more likely heat would be *gained* by reflective radiation from the ground, not to mention solar energy.
That leaves evaporation, which is the major mechanism for human bodies to cool. It requires the evaporation of water. The evaporated water carries the heat away into the atmosphere. If the water is condensed, the heat of evaporation is returned to where it is condensed.
Thus, a "stillsuit" receives the heat the body has dumped, and has to release that heat to the environment, without giving it back to the body.
I suspect clever engineering with a compact, high energy source, could do it, perhaps by radiating the heat from a very hot radiator, shielded from the body. But, it requires a lot of energy to do it.
You cannot do it with muscle energy from the same body you are trying to cool.
It is just another suspension of disbelief, piled on many others in the novel. Good science fiction has one, around which the story is built.
I suppose it was a good fantasy novel, with quite a bit of magic disguised as technology.