Ken and TG

This story makes perfect sense to me. Wood rot is caused by fungi. Before fungi can colonize wood, four requirements must be met: an oxygen supply, temperature in the 40 to 100 (F) range, a supply of sufficient moisture, and a food source (wood). Infection can be prevented by eliminating any one of the requirements. What has happened in this case is that the posts rotted at the margin where they enter the ground and are in contact with the soil but still have access to oxygen. The bases that you found were cut off from the oxygen.
I have seen this phenomenon with WRC fences before. The post has rotted off and you think you are going to replace it where it was, only to find the old post base intact in the ground.
It should also be noted that the tannins in Red Cedar that make it rot resistant accumulate over time so that old growth heartwood is vastly superior in rot resistance to much of the cedar available now.
My bet is those post stumps are as old as you think. Unfortunately there is no lesson here for building because there in no way to avoid the rot at the transition