Sun is MUCH more intense actually so protection is paramount - at these elevations you are getting absolutely blasted in much higher amounts of UV - and on top of that snow also reflects tons of UV and light which makes it even worse. They do these climbs of course in summer when the weather windows open so this is a seriously intense problem. Quality glacier glasses / lenses really opened up this whole world immensely - up until this point people had to fashion weird things like putting little slits in clothing and covering the face and still tons of people go snowblind. Based on my (limited) experience with mountaineering I would guess that while sunscreen is most likely a factor they are covering up their faces with high UPF clothing.
Someone else will be able to give a scientific explanation, but the sun is just different at heights. I used to have famiy both in La Paz and Quito and just 15 days in one of those places -- dressed for cold --made me as brown as two months going to the beach in coastal Brazil.
Sun at altitude is much, much more potent because of decreased atmospheric protection you get from UV rays. UV exposure goes up by about 4% every 1000ft increase, so living in Denver I get about 20% more UV exposure than someone at sea level!