[FishInTheWater]

For personal ethics, they are mere opinion as you get to choose what those ethics are. For professional ethics, those ethics are the opinions of the relevant professional associations and regulatory bodies.

They are facts in the sense that "The Bar believes that it is unethical to lie to the court" is a fact. It is simply factually true that the legal profession holds that ethics belief.

And thus the point, anyone who seeks to join such a profession has to accept their ethics "as if" they were facts. You can't choose other professional ethics in the sense that you can choose to hold a different opinion.