I find that's a bit too handwavy.

Why would you redefine something as fundamental as a fact, that's quite different from an opinion, as "the opinion that a group enforces"?

The repercussions if you violate those opinions might have factual consequences for you, but that makes the basis for all this not more factual.

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 ethical 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.

Minor detail, but the author is female.

Thank you, my bad, moved to gender neutral “they”.