"Copyrighted" refers to the work. "Infringing" refers to the *use* of the work. The first does not imply the second.
The aricle says they checked "...whether the file was confirmed to be copyrighted..." And then apparently made the jump to assuming that anything copyrighted must be illegal, sliding immediately into called them "infringing files."
Of course by that metric all the Linux distros are illegal as well since they too are "copyrighted." As is any blog post, web page, or photo taken in the last, say, 70 years. As is anything that is shared properly according to the terms of any license. Now the study may have actually looked at the license terms in place for each work, but this definitely not what the article *said*.
Not to mention that regardless of any express license terms, sharing that qualifies as fair use is also NOT AN INFRINGMENT and is LEGAL and should not be described as illegal or as "infringing files."
Any indication whether these types of things (terms of the licenses according to each item, whether the sharing events qualified as fair use) were taken into account? If not, then I'd counter by noting that 100% of the material on Warner Bros' home page is copyrighted too. Should I say it's being shared "illegally"? Of course not, but my whole point is that if you play with semantics loosely enough, you'll find that probably the vast majority of the material on the Net as a whole is "illegal" and "copyrighted."
*grumble*