(Read more: Ackman's latest letter)
Ackman has also pushed to more quickly replace CEO Myron Ullman, who was brought back in April to stem sales declines blamed on Ron Johnson, Ackman's pick to turn around the struggling century-old department store chain.
In an earlier letter, made public Thursday, the billionaire hedge fund manager behind Pershing Square Capital Management expressed frustration at the slow pace of the CEO search and pushed the retailer to name a top executive in 30 to 45 days.
(Read more: JC Penney board erupts into fight over next CEO)
To Schultz, though, the irony is that Ackman had a heavy hand in the retailer's recent struggles.
"Ackman was the primary engineer and architect of recruiting Ron Johnson to the company, and he and Ron Johnson pulled off this strategy that has fractured the company and ruined the lives of thousands of J.C. Penney employees and fractured shareholder value," Schultz said, adding that he's confident Ullman could turn things around if only given more time to perform.
Ullman serves on Starbucks' board. Schultz disclosed that he is not a JCP shareholder and has never met Ackman.
—By CNBC's Drew Sandholm. Follow him on Twitter
—Reuters contributed to this report