> it means your purchasing power with respect to cheap commodities tracked by the CPI is preserved

CPI isn't a measure of commodities. And "CPI" is a bit of shorthand, given there are pretty much as many measures of consumer and producer prices as there are economists.

> it doesn't necessarily mean "value" (whatever that means in the abstract) is retained

This is what any measure of inflation ultimately seeks to measure. Purchasing power is intrinsically tied to the basket of goods and services its measuring. That basket varies across people and time as preferences vary.