IME, the vast majority of koi foods on the market consist of what I would personally consider excessive amounts of terrestrial based plant matter. That corn, soybean, wheat, etc may be acceptable for a fish such as a koi, that has no stomach, and that can typically assimilate various raw ingredients, and carbohydrate levels, that many tropical species of fish cannot. But not acceptable to me, personally, and the fish I keep.
But there is a massive difference in what may or may not work on a commercial farm, or what is going to be most cost effective, on a commercial farm, vs 3 or 4 tanks in the average hobbyists basement.
I had this discussion with Pablo Tepoot, the owner of New Life Spectrum, several years ago when we were talking about what he fed his fish back in the day. Back when he had several millions of gallons worth of ponds, and concrete vats. This is a man who had his own line of food, but couldn't afford to feed it to his own fish! It just wasn't cost effective to feed 10's of thousands of pond fish, primo food. Now that he is producing the food himself, with his own equipment, on site at his farm in FL, that no longer is true. But for many many years he fed bulk farm feed, just like Don.
So a guy like Don, with the size of his operation, isn't going to be feeding NLS, or Northfin, or Omega, or anything close. He's feeding a "high protein" food, most likely designed for trout, mixed with a bulk commercial koi food. That makes perfect business sense, and the fish would also be consuming natural pond algae, insects, larvae, etc, as a supplement.
Waste isn't near the factor in a large vat, or natural pond setting, but is certainly a concern to most hobbyists, and the larger the fiber content, the more solid waste that will be produced. The key in feeding fish beyond just nutrient levels, is digestibility. Higher quality allows one to feed less, which means less waste, less bacteria, and less of a bio load on the entire system.
In large aquaculture facilities the feed conversion ratio is constantly scrutinized, getting it right can mean the difference in hundreds of thousands of dollars spent, or saved, in feed.
I have no issue adding fiber to the diet of species that require additional fiber, I'm just saying that there are better ways to do it than feeding your typical grain loaded koi food.