It's a brave Commander who overrides a cabin crew member who has stated that they are unfit for discretion, and thereby, unfit to fly.
The OMA quite clearly, and separately, places an obligation on each and every crew member not to fly if they believe they are, or may become, unfit to fly during the rostered duty.
Far better to see how many crew (if any) can be stood down, while still being able to operate with the remaining crew.
Imagine a botched pax evac after landing, caused by a cabin crew who had been told by the Commander that they were fit to operate into discretion after they had gone on the record stating that they were not.
Commander's discretion is all well and good but you are potentially opening up a whole world of legal pain if you override a crew member who says they are not fit to operate.
It is invariably a balancing act between assessing the fitness of crew who may be stating they are unfit for less than genuine reasons and fighting commercial pressure from an employer whose first priority is to get the job done and avoid hotac costs and roster disruption - with no consideration whatsoever for fatigue or safety.
Not easy.