My guess is that they're lying. Because a fib costs them nothing and is hard to verify, given that someone who objects to Rush's program is not going to spend hours listening. Someone who supports Rush's program and hears from Rush that such-and-such a sponsor has cancelled is going to stop giving that sponsor his business. People like the Carbonite CEO don't realize that there is a major difference in commitment between people who listen to Rush and people who don't. Non-listeners would never patronize a sponsor's business just for boycotting Rush, whereas listeners will boycott them in retaliation.
Then why make a public statement. Just call the advertising company and let them know they made a mistake. They entered this when they did not have to do anything publicly.
Now they need to make a public announcement that they did not pull any advertising and ask Media Matters and others to amend the list without them on it.