Perhaps with the work of jveritas and others translating the Iraq document dump, the WaPo knows it's just a matter of very little time before the truth comes out. When faced with the inevitable, they may believe that it's best to now get on the side of the truth.
You may be right for all I know. That sure hasn't stopped them from harping on non-truth in the past. It's also possible that they have something they'd like to spring in a few weeks, and want to lay down a shred of credibility before they do so, as unlikely that prospect may be.
Why whould they be starting now?