Your initial assumption was correct, it is widely deployed in the industry, there are multiple vendors that provide fingerprinting technology that is orders of magnitude more advanced then what the article suggests.
The visible fingerprint might just have been there to deter the clueless. In any case even with a fingerprint its hard to proof without a doubt, the individual who actually stole it.
The visible fingerprint might also be there to prove intent or add circumvention charges in court.
I don't think they need to prove that a certain individual stole it. The party which received the advanced copies likely had to take on liability, so HBO can go after them. Of course, that doesn't mean they won't turn around and lean on the actual culprit, but whether or not they catch someone HBO will have their due.
The visible fingerprint might just have been there to deter the clueless. In any case even with a fingerprint its hard to proof without a doubt, the individual who actually stole it.