Our Own Little Secret

The New Yorker magazine recently had a piece about how hotel and casino magnate Steve Wynn put his elbow with a Picasso painting that was worth $40 million. Thankfully for him he owned the painting. But he was in the process of selling it. There were already promises on the table from Wynn to sell and from the purchaser to buy. But that's not the interesting part of this article, as it's clear that the buyer would has a remedy to get out of the contract. Here is the interesting part:

A few hours later, they all met for dinner (Wynn and his friend who witnessed the accidental destruction of the painting), and Wynn was in a cheerful mood. “My feeling was, It’s a picture, it’s my picture, we’ll fix it. Nobody got sick or died. It’s a picture. It took Picasso five hours to paint it.” Mary Boies ordered a six-litre bottle of Bordeaux, and when it was empty she had everyone sign the label, to commemorate the calamitous afternoon. Wynn signed it “Mary, it’s all about scale—Steve.” Everyone had agreed to take what one participant called a “vow of silence.” (The vow lasted a week, until someone leaked the rudiments of the story to the Post.)

Since the New York Post broke the story of this $40 million blunder, Wynn's guests have been speaking with whomever will listen. But they promised not to talk about it. Is this an enforceable promise?