Why would he (Cheney) be scared? The most plausible inference is that he knew he had deliberately rigged the WMD evidence to ensure that the war took place. He knew, even if the president was blithely convinced otherwise, that the WMD evidence was weak, and his success in distorting the evidence was threatened by Wilson. Not that Wilson had all the goods - Cheney must have known this was a minor matter. It was the danger that journalists or skeptics pulling on the thread that Wilson represented could get closer to the much bigger truth of WMD deception. This is a huge deal for one single reason: if true, it means that the White House acted in bad faith in making the case for war. There is no graver charge than that. In fact, if true, it's impeachable.
At greater length here
This still does not excuse Sullivan's original support for the war and a whole bunch of other rightwing idiocies on his part, but I am intrigued that the impeachment meme has moved so far into discourse.