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.