You're missing the point entirely - this horse is deliberately standing "under himself". His hocks are not lined up to his butt, which gives the impression of being sickle hocked. You should see a straight line from the point of buttocks to the point of hock, and then they would slope forward from there.
The fact that his hocks are so far forward make it blatantly obvious he's just standing like a knob.
This is a sickle hocked horse:
As you state, the only way to tell without physically measuring the leg bones is to line up the hock with the point of the butt...for some horses that is a natural stance, but again many horses' natural stance is camped under slightly which gives the false appearance of being sickle hocked...