From the article, "...a lot of smart people have been questioning the accuracy of our 90% estimate, and with good reason, it’s a very rough estimate and the measurements are flawed."

Sorry, wasn't able to find that from the original link. I take back my last comment.

I guess it boils down to how many of those pirates you think were ever potential customers. Assuming piracy was theoretically impossible, how many would've purchased it rather than not play at all?