I am no expert but sounds like they knew what they were doing :) Some drafts do have a condition where if their diet is too high in protein and not enough fat, they develop:
EPSM--equine polysaccharide storage myopathy--is a condition in which a horse is unable to process and burn the fuels provided by starchy grains. The list of EPSM signs includes symmetric loss of muscling, especially of the rump, shoulders and topline, weakness, usually in the hindquarters, trembling, often after exercise, difficulty in backing/reluctance to back, poor performance, lack of energy, stumbling, reluctance to pick up the feet for farriery work, a slightly stiff, awkward or short-strided gait in one or both hind limbs, “locking” stifles or “shivers,” in which a hind leg hikes up and out and possibly the tail also rises and quivers, episodic colic-like signs, especially after exercise, tying up, inability to rise from recumbency.
The diet change involves decreasing the amount of dietary carbohydrates and replacing them with fat as an energy source.
I have a draft cross and have always fed him grass hay and a high fat supplemental pellet (Purina Strategy) and flaxseed meal. He does great, maintains his weight and whether he has the potential for EPSM or not, I'd rather play it safe :)