That article does not say that TPG wrote off the whole investment on WMI. It just states that they wrote down only %29% of the fund which includes several investemnts of TPG and only one of them is WMI. And their write down on WMI loss is only $473 million. And also we do not know what is the portion of WMI on that fund they wrote down %29 off:
"..........NEW YORK, March 18 (Reuters) - Texas-based private equity firm TPG wrote down the value of investments in its latest buyout fund by 29 percent at the end of 2008, according to a document obtained by Reuters on Wednesday.
TPG Partners VI is a buyout fund of about $19 billion in size which the firm finished raising 6 months ago, so only a very small percentage of the fund has so far been invested.
One of TPG's high profile investments, in savings and loan company Washington Mutual Inc, was wiped out by a move in September by the U.S. government to close and sell WaMu's banking assets to JPMorgan Chase & Co (JPM.N).
The fund reported a realized loss of $473 million on that investment, according to the document......."