Inflation is a measure of the growth in supply of money.

Using purchasing power (or more specifically an artificially constructed and highly manipulated bag of goods measure called the CPI in the USA) is a switch to obscure the rate of inflation, as if it couldn't be measured directly.

But Inflation always has been issuance, not purchasing power. Pretending otherwise isa tactic used by people who engage in inflation to try and obscure its effects (because purchasing power is affected by things like growth in productivity.)