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.

If you're talking about simply copying the files to a CD (as iTunes suggests after the files are downloaded), you'll lose access to the data if (when) you switch computers and their authentication system no longer provides you with the key you need to play the files.

If you're talking about burning them as audio tracks on CDs that can then be re-imported to whatever format you want, for many people that's not an acceptable solution: it requires you to use many more CDs than simply storing the compressed files (and what happens if I don't want to use CDs at all, but instead backup to some other HD or remote service?), and re-importing it will either cause further quality loss or occupy a lot more space than the original files.

Personally, I only started buying DVDs after CSS was broken, I only buy music from iTunes because QTFairUse is available, and I'd only consider buying encrypted PDFs because there are programs that remove their protection. There are some videos there I'd also like to buy, but I'll only do it the day a program that can remove the DRM from them appears. I prefer buying content in an unprotected format if at all possible, but when it's not, my policy is to never buy any protected content from which I can't remove the protection without losing information.