Well, it doesn't make any sense to me. A horse is physically designed to eat with its head down on the ground. I would bet that the muscles work better that way, the esophagus is better extended and who knows, maybe affects the way the food is worked in the mouth too.
If anyone else has heard this, please post an explanation. I know from my experience with my x-track horse (see above) I have had success with ground level feeding.
Oh just thought of something -- maybe, N66, you read about not feeding a horse from the ground due to sand colic risk? That's a different issue, but we do read so much stuff it's easy to get inundated!