ABA Formal Opinion 06-439 discusses "the obligation of a lawyer to be truthful when making statements on behalf of clients in negotiations, including the specialized form of negotiation known as caucused mediation." The opinion allows for "posturing" or "puffing" by parties to the negotiation. This includes understating their willingness to make concessions and exaggerating their strengths. These are not considered false statements of material fact under Model Rule 3.3 or 4.1.
Formal Opinion 06-439: Lawyer's Obligation of Truthfulness When Representing a Client in Negotiation: Application to Caucused Mediation
ABA Standing Committee on Ethics and Professional Responsibility. Formal Opinion 06-439, Apr. 12, 2006https://www.americanbar.org/products/ecd/chapter/219913/