The "free turbine" is in reference to the engine and not the powertrain as a whole. What it means is the gas generator and power turbine are "gas" coupled and mechanically coupled.
The freewheel or overrunning clutch or sprague clutch is the unit which decouples the transmission from the engine. The decoupling is necessary so the engine will not be driven by the transmission. If the engine were driven by the transmission this would use up rotor system rpm and not allow sufficent rpm to complete the autorotation.