I think the Kubby coolant is weaksauce on the corrosion inhibitor concentration.Interesting.
Why such a short coolant change interval then? Even pre extended service interval cars (30 years ago) weren't that frequent. And these don't have aluminum heads on cast iron blocks.
(Not arguing, curious)
Behold: https://www.orangetractortalks.com/forums/threads/change-coolant-on-tiniest-of-tractors.62097/