I checked both this article and the articles referenced in it and could not find a single place where they back that number up with any hard research. Willing to bet there wasn't any.
edit: source of the 90% (or revised 82%): http://2dboy.com/2008/11/13/90/ (sorry bsphil!)
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?