The point was that the Carnot efficiency of using a thermal buffer to store mechanical energy cancels out between storing and releasing. Yes, you don't have a

The point was that the Carnot efficiency of using a thermal buffer to store mechanical energy cancels out between storing and releasing. Yes, you don't have a Carnot-cycle in practice, and the thermal buffer likely won't offer isotherm storage, either. But you still can't use Carnot-cycle efficiency to determine some sort of round-trip efficiency of this thermal-based storage device with mechanical input/output ports.