They explain that: because professional ethics are enforced. If some licensing panel finds you violated the documented ethics, your career in that field can be over… for life.
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.
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.