From the article:

Then, even if the data is encrypted with an unknown key, we can expect almost exactly half the clusters to be chained to consecutive ones, and they are distributed a random fashion. By counting the length of consecutive cluster blocks, we should see that 1/2 of them have 1 cluster, 1/4 have 2 clusters, 1/8 have 3 clusters and so on, and they are evenly distributed along the drive.

It's very unlikely that such a distribution would appear spontaneously on a disk by just using it normally, so someone who knows that this scheme exists can check whether it is present on the disk, even if they're not able to decode the data.

(Disclaimer: I haven't read the actual paper, they may have addressed this. Or the claim in the article may be incorrect.)

I've been worrying about the move to online content for many years now. With regular CDs and DVDs, if the content you want doesn't have a distributor in your region, it's usually easy, if a little more expensive, to buy it from a foreign store. With digital content you don't have that option: if the content is not being distributed by anyone in your region, you have no choice, no matter how much you're willing to pay (unless you're willing to work around the terms of service, or if you move to the other country and consume it there). The internet, which has the potential to make any content accessible to anyone in the world who's interested, instead ends up making the geographical divide stronger.

I'm a completist music freak, so I get really mad when there are digital-only releases or bonus songs I cannot get. I understand that most of the time the artists have no responsibility and don't even realize this is happening, so when I have the chance I try to point it out to them; but I haven't had even a single acknowledgement so far, and I'm starting to wonder if anyone actually thinks this is a problem.