Are you maybe thinking of what MMTers e.g. Warren Mosler assert? (E.g., Mosler calling for 0% interest rates, always.)

If you can think of source, please let me know. Sounds interesting.

Edit: It also sounds fallacious to me. Interest rates are central-bank determined because the central bank chooses to determine them. In the absence of a central bank controlling rates, there undoubtedly would still be interest rates. There ostensibly also still would be risk free rates. In some hypothetical parallel Earth, the Fed might instead choose to control the price of some other commodity, like oil. That doesn't mean that the price of oil would be "endogenous" and therefore that there's no market price. Just that the Fed had chosen to suppress that market price. Thus, as far as I can tell, it still makes sense to ask the question "what would be the market risk free rate?"