How do they even know that?
They had an option to send your score to a central server. Everytime someone did that, there was a check to see whether the game was legitimaly bought or not. 9 out of 10 games sent were pirated.
Actually, they corrected their figures, because people said they bought the game once and copied on several computers, so it was by their estimation closer to 6 out of 7 or 7 out of 8.
Being an indie, I think World of Goo benefited from the piracy. But if you take the same numbers (dubious method I know) and applies it to other games "not indie" who do no profit from piracy, and assume that only 5% of the pirated game would actually have been bought if not pirated, that means (for a very conservative 4 out of 5) +20% games sold, which is enormous given the nature of the product (huge fixed cost).