Just because a dog stands with its rear pastern sloping forwards does not necessarily mean it has sickle hocks.
In simple terms a correctly angulated dog cannot not stand 'sickle hocked' and an overangulated dog can and will.
On the left is a correctly angulated dog who by that definition will not stand 'sickle hocked'. Note that the front of the rear toes are about a fist wide behind a vertical line dropped from the tail root when the rear pastern is vertical. This dog can stand with the rear pastern sloping forward of course, but to do this the foot will be under the vertical line not behind it. The determination for sickle hocks is made in this very precise configuration.
Then we have the next diagram which is of an overangulated dog. You can see that when the rear pastern is placed vertically the foot is a 'long way' behind the vertical line. The tibia, the lower thigh, is now almost parallel with the ground and the hip and knee are too close the ground. This dog, because it is overangulated will stand with 'sickle hocks' and this is shown in my next very rough diagram that I knocked up quickly this afternoon.
Here you can see if you pull the rear foot forward to get the toes closer to the vertical line which is the determinate for ascertaining correct hind angulation, the rear pastern is sloped forward - this is a 'sickle hock'. As an aside because this is the same hindquarter as the previous dog/diagram the other changes that take place when the foot is located forward is that the femur and tibia open out/straighten and as this happens the pelvis/croup rises and becomes less angled.
You can see why the term is never used in the GSD sport because once you declare a dog as being overangulated in the hindquarter it will ''in a natural stance'', not a forced stance but a natural stance ''for reasons of balance overangulated dogs stand sickle hocked to bring their supporting rear foot closer to the point of the the rear hand weight which is in a vertical line just under the hip and knee'' and if you really look, you will often see this in deeply overangulated dogs when they are standing naturally.
I hope that helps, and thanks to Rickard for posting me a photo of a dog that is 'sickle hocked'.