Good skiers for sure, but they are not doing slalom turns but more like glalom/GS across the slope - not the same thing... The last one is not bad - I don't know him... but he's specifically doing and talking longer turns - hence the title of the video... While the middle one says "big turns" - I guess they both know what they're skiing....
Uh... It's not as exhausting as it may look, depending what you ski, although I guess if you want performance you have to put up with it. I know Greg skis with big angles all day, even after slaying the steep icy runs at hip-to-snow. Blue jacket skis with big angles all the time out of the trees and he doesn't do this thing you call gym. The trouble is that once you get there, skiing at the "normal?" angles is not that much fun anymore, I guess...
If it's steep and hard, you have to put up with a lot of energy, so generally the way they tend to back off is by skiing more mellow terrain, I guess.