It’s the near-vacuum, plus the radiation environment, mostly. Temperatures on Mars can rise above freezing in the tropics, on a particularly warm summer day, but the average temperature on the planet is several tens of degrees C on the negative side. The author’s point is, I believe, that even in the southern polar night a human being can survive, for a while, with stone age technology. Whereas if the atmosphere around you isn’t breathable there’s not much you can do without (literally) space age gear.
Wow, on the surface pictures made by the rover, it looks like a hot desert. I understand that the lack of water causes this drastic difference in appearance, but never would have imagined the difference to be so big.