The significance is that Woodward admits that he may have passed that information on to Libby, indicating Libby may only be guilty of a faulty memory and so over confident in his innocense that he hadn't bothered to review his own notes before testifying. Forgetfulness and over confidence is not perjury.
Libby's lawyer will try to make that point, however that is not what Woodward said. He said he didnt recall asking Libby about Wilson, he said if he had he would likely have recorded a note next to the question (there was no note). He had no reason to tell Libby about it. It wasnt something that was important to him in the preparation of his book. The clear point of his statement was that it was highly unlikely that he would have told Libby but since he has no recollection of it, he cant be certain to a metaphisical certainty. However, that only covers half his problem. He still needs to be able to explain why something that apparently so important to him and to the administration that he could forget to tell the FBI and the grand jury that Cheney, two CIA people, and a under secretary at the State Department told him earlier about Wilson and Plame both verbally and in writing. He also neglected to mention or recall that he had participated in meetings with three other administration officals including Ari Fleischer where Wilson and his wife came up in conversation. In case of Fleishcer, Libby told him that wilsons wife worked at the CIA on WMD. Libby still has major problems.