If you have a pond in the range of 1000 gallons and up and you have a great bio filter that is free flowing and easily cleaned and you have good circulation in the pond (no dead spots) you can push the envelope on how many fish you can have in your pond. By the way, big rocks in the pond create dead spots. I have never lost a fish in over 40 years and my pond is overstocked if you believe these articles.
That said, these articles are written because a lot of ponders that don't have larger ponds with good filtration, circulation, and areation are convinced they have to have a lot of fish to make their investment payoff. These are the targets of these guidelines.
I go to one the local importers of Japanese Koi to visit regularly. His tanks have 50 times the safe amount in them and the fish are in better health than most ponds. Why? His water is moving through massive filters at a rate of 10 water exchanges an hour or more. I have never seen more lively Koi than at his warehouse.
So if you have a tiny preformed pond, heed the advice in these guidlines, if you have a large pond with all the above working for you, use your head when buying that "one more koi" to add to your collection.