The Autopilot is flying the aircraft. The FMC provides mach trim and speed trim, hence the comment about performing CAT III autoland W/O FMC is valid. (as long as the AP is fully functional)
The pilot will have to manually trim the ac if the FMC's are inop. and can still perform a CAT III autoland procedure.
Is the question how to manually trim the ac below 400' on final???