With the benefit of hindsight and knowing what actually happened, it's easy to say they should have said something day one even if they weren't sure. But people should probably also consider what a disaster it also would have been if the situation was reversed, and their premature statements caused people to needlessly panic - it would have been two rounds of PR disasters instead of one, and they would've had to be dealing with the PR side of the issue at the same time as the technical instead of being able to determine extent of damage first. The two should be separate, but invariably there will be an impact.
It would obviously have been nice to have both, but when it comes down to speed vs accuracy in cases of data breach, I can't say that making the decision to go with the latter, if that's what they did as the last PS Blog post mentions, is the wrong one.