The problem with the metallic definition, meaning the definition of "inflation" from the roman empire until the mid 1900s, is that it didn't really work well with Keynesian economics or modern monetary theory.

Inflation has a bad connotation historically due to the number of examples where increasing the money supply too quickly ruined economies and destroyed empires. MMT and Keynesian economics use the money supply as the primary tool for controlling the economy.

They may not like that "inflation" described the exact mechanism for the main tool of modern economics, but that doesn't make it wrong. The easily could have come up with a new term for an increase in the price of goods rather than co-opting an existing term. That strategy seems very much like a play driven by ulterior motives.

It isn't so much that we had to understand new concepts as it was they had to redefine terms to put their new game in a better light. That's also why they talk about price increases rather than currency devaluation or theft. Both would be accurate, but price changes sound more benign.