The crew decided not to go into discretion?
No, it is Commanders discretion. He evaluates how well rested/capable his crew is to complete the duty, and then he makes his decision.. full stop. When he asks you at dispatch how "busy" you are at the moment, or what time zone you are on, that is the opportunity to tell him what to expect from you. Note, CX expects that you must turn up fit and rested for the rostered duty.
It is almost impossible for the Company to question the Commander on this decision. If anyone else from the crew is questioned, then their response should concentrate on this point, after calling the HKAOA.