It doesn't matter what the rules say right now, his wealth is at risk. They can and do change the rules sometimes. They can change what the word "inflation" means. Maybe someone legislates that inflation is 1% now for paying back I-bonds, I dunno. This is risk, it could happen. Something is going to happen, the debt situation is unprecedented bad. Will that something affect these bonds? It is possible.
We’ve all heard about shrink inflation by now. Theoretically that’s measurable by taking the Oz. into account.
But how about the drop in quality of goods? Raisin Bran with few raisins, cars with thinner door panels, etc. This is impossible to measure, obviously happens, and is systematic in one direction resulting in a lower reported inflation rate.
Moreover, what they're attempting to match isn't inflation, it's a specific government inflation metric which contains many corrections which make it obviously not match inflation for the purpose of preserving purchasing power.
For example, when inflation goes up the metric assumes that purchasing will shift from high value goods (like steak) to lower value goods (like rice), which causes the metric to read lower. This is no good if your goal is preserving purchasing power: you put in enough to buy steak and the inflation metric is okay with what comes out being only enough to buy a meal of rice (not technically, since steak and rice aren't the only goods but you get my point).
