Although domesticated horses are far removed from their original state, my philosophy is that we should endeavor to try to do what is best and natural for them ( as much exercise as possible, keep them in a herd environment, provide most of their daily ration as forage, etc.)
So... Is coconut the closest thing to nature for them? That is the common sense answer I'd ask myself first. If yes, then I'd look into it further.
But I don't think that's the best fat source for them... Even if they do market that coco soya product. If anything, I'd try that first....