Nah, no worries there.
They arent forcing themselves in that position or by adding external weight. They lower themselves in a certain , natural shape. Compare it with an infant squatting entirely down, thats also a natural shape.
Why cant adults ? Flexibility. But these are professional hockey players. Big emphias on muscle conditionning, posture and flexibility. Especially flexibility, so they arent stressing themselves.
Anything done on bodyweight that is not forced and aided by the recommended flexibility training, will be just fine.
Gymnastics ? No, thats an insane stress on the joints. You're done at your early 20ties generally, sometimes before that