Cohen has ethical problems, but this is probably pretty far down the list.
If he were lying about the law, though, that could be treated as a violation of Rule 4.1 of the New York Rules of Professional Conduct:
In the course of representing a client, a lawyer shall not knowingly make a
false statement of fact or law to a third person.
It could also be a violation of Rule 8.4:
A lawyer or law firm shall not ...
(c) engage in conduct involving dishonesty, fraud, deceit or
Although these would be violations of his ethical obligations, they probably do not rise to the level of seriousness that would result in any meaningful punishment. Disbarment would be extremely unlikely, though a censure is conceivable. If someone reported the offense, I would actually expect that the state would decline to investigate at all.
Of course, all of this assumes that he was deliberately lying about the state of the law, which I think overestimates his competence. More likely, he's just an idiot and didn't know that he was wrong.
Importantly, being wrong about the law isn't unethical; it's a presumption at the foundation of our adversarial justice system.