Kevin, I don't understand why you ask this. It's simple. The media is degrading and they can not read the files. We are not talking about a DVD commercial copy. The medium they store the data could be magnetic like VHS. (like D-VHS) Your DVD commercial copy could also be damaged and stop working.
I read the article. It means that some of the storage medium that contain digital files are degrading faster than film. Completely understandable, like a DVD-R that you burned 5 years ago and can't read it again!