Mondeoman:
You are half right, indeed the RETARD auto-callout is a reminder that you should be at idle by the time you hear it.
The most important thing to know is as you said, that auto-thrust will be active and chasing the target speed (either selected or managed) until the thust levers are manually selected to idle; even if you are flaring, this will lead to unwanted acceleration and power increase. Pilots are supposed to use the same airmanship as any other airplane to retard the thrust levers, not wait for instructions.
Where you are wrong is that the same applies to auto landings retard is not an order it is a reminder that you should be at idle, only with the added protection that the power will automatically come to idle if the thrust levers are not retarded.
barke:
It doesn't work this way on Airbus FBW because of the logic of the auto thrust, you must bring the thrust levers to idle for touch down, otherwise the speed will increase during your flare to maintain selected or managed speed.
G