On the contrary. The fact that you couldn't take that position while being honest, proves that morality is objective, and you, like most people, feel it instinctively.
Just because you can state a wrong moral postulate, doesn't mean that it can't be proven wrong.
I obviously don't agree with the moral postulate I stated, but
that was the entire point, to show that there are ones that neither I, nor (hopefully) you, agree with. But that there nevertheless exist ones that
do agree with it (ie. a government or monarch with "divine right/god's mandate").
Had I agreed with the postulate myself, then I would have only reinforced the possibility of it (good/evil) being objective (making the entire exercise redundant). What was important was that we disagreed with whatever I postulated, and that
someone agrees with it (as that means it is subjective by definition).
But if you want an example grounded more in reality (despite being identical to the one in this chapter), we have terrorism: where the perpetrators consider it a good & justified act for a good & just cause, while the victims and surroundings tend to consider it pure evil.
ie. A disgraced saint labeled as fake, attacking the prince of a nation to kidnap him from his seat of power. Fighting the true saint in the process.