I couldn't quickly find a definition of "commercial piracy", so I will assume it doesn't have a specific technical meaning.
Facebook is a commercial company, so if you're working for Facebook doing piracy, that would be commercial piracy.
If I go by piracy as "the unauthorized use of another's production, invention, or conception especially in infringement of a copyright", and if Facebook is infringing on copyright in using these books (which the article alleges), then it follows that Facebook is engaging in piracy.
So it looks like Facebook is partaking in piracy of a commercial flavor.
I don't know why distributing bootlegs to people on the street would be worse than distributing PDFs within a company, but let's say it it. We are talking about 100s of cases vs millions. Is distributing on the street really 10000 times worse?
It would have to be even more than that. Because millions of copyright infringement within a company apparently has no punishment, but 100's on the street has prison. So, if you go by the punishment, 100's on the street is way worse.