> Money supply is always estimated. Modern monetary systems don't have a single person with a monopoly on creating money.

That actually seems like the sort of hand-wave that the accounting industry exists for the sole purpose of opposing. There are a small number of groups licensed to create money by lending, and it is likely they have very accurate records that are to be made available to financial authorities.

It may be that the central banks don't bother to use their power to get an accurate figure, but there isn't much of a question that the quantity of money emitted by the system is a precisely known figure. And most of it only exists electronically.