ADP's board announced its rejection of the nominees Monday. During a recent interview on CNBC, ADP CEO Carlos Rodriguez likened Ackman to a "spoiled brat."
But ADP said Ackman told company that once he nominated directors, he would be committed to a proxy fight and would not settle. And, the ADP disclosure said, Ackman added that if he were engaged in a proxy fight, he would "use his ability to generate media coverage to damage both Mr. Rodriguez and the company and that would be bad for the company's clients, employees and stockholders."
A spokesman for Pershing Square said declined to comment on Wednesday other than to point to its proxy statement, which didn't mention the talk about Ackman's ability to cause media coverage but did say Ackman told ADP it was in the company's interest to avoid the distraction of a board fight.
Ackman offered ADP an olive branch of sorts on Thursday in a letter filed with the Securities and Exchange Commission.
"As often happens in these types of situations, it appears that there have been misunderstandings over the past few weeks which unfortunately have clouded what should have been a productive relationship from the start," the letter said. "I am sure that you would agree that it would serve all stakeholders if we can put these issues behind us and focus on what is in the long-term best interests of ADP."
Ackman is scheduled to meet ADP's board members on Sept. 5.
From ADP's perspective, Ackman wanted an extension of time because he was going to be on vacation.
According to ADP's proxy filing, the exchange between Rodriguez and Ackman went something like this:
"Mr. Rodriguez asked Mr. Ackman if he seriously expected a Fortune 500 company to change a published deadline so as not to interfere with his vacation, to which Mr. Ackman replied, 'I'm dead serious.'"
Before the latest proxy, Pershing Square told CNBC that ADP's account, including its claim about Ackman's vacation, is"totally false."