Those "gallons per inch of fish" rules are from the aquarium world. In no way does that apply to keeping koi. (I can't even say for sure it works in aquariums, but that's not my point right now.)
Here's a chart I like to share with people to help illustrate this.
This chart also lists koi weight by weight and how much ammonia and waste they produce. Koi get heavier as they grow and produce more ammonia and waste.
russellwatergardens.com
In particular compare the difference between the waste produced by a 3 inch fish and a 30 inch fish. If the plant"inch per gallon" rule was legitimate you would expect the difference to be ten times, right? Instead it's a factor of ONE THOUSAND. Put 10 three inch fish (which is 30 inches of fish for your formula) in a pond and you'd have 24 grams of waste produced per 24 hours. Put one 30 inch koi in the same pond and you'd have 2,449 grams per hour of waste - tremendous S.
The biomass of a big koi increases exponentially as it grows. I've never found a reference guide that claims to accurately be able to tell you how many koi you CAN safely keep in a pond. To me, the answer is always "far less than you'd like to".
Again - there are experienced koi keepers who understand water quality and are willing to do what it takes to keep a too small volume of water safe for too many fish. But that requires extensive filtration and constant monitoring of water conditions. You can't do that with your standard biofalls, sponge filters, and lava rock.
I too have seen many too small ponds inside and out in commercial spaces - here's a secret: they probably swap out their fish on a regular basis. Those fish act as ornaments, not pets. They have commercial services that provide them with fish, just like they have services that provide them with plants. I learned that at a local Japanese hibachi restaurant when I asked that very question - how do your fish stay healthy in that small pool? "Oh we get new fish all the time".