Must be pretty cold. I’ve been hiking in areas that still have deep snow and even then I took off all my layers and ended up in a tshirt. Only thing I can think is maybe this was either at the start of their practice, or they practiced for a very short time (maybe just for the video) and as such didn’t have time to get warm? Because when I was younger we’d wear sweatshirts and warm pants to morning gymnastics practices, and take them off as we got warm enough, meaning that at the start we’d be exercising in warm clothes.