Link to the decision:
http://www.ustaxcourt.gov/InOpHistoric/ADInvestmentDiv.Halpern.TC.WPD.pdf
Attorney Client privilege has never been absolute. There are a variety of reason why it may not be valid. As I read it, the client has implicitly waived Attorney Client privilege because they have argued their attorney gave them X advice, and thus the other side gets to see if the attorney actually did give that advice.