1 & 1 had a little feature on their servers called "URL correction", where if you mistyped a URL it would automatically correct it for you.
I despised this, because if you were trying to hide files, it would expose them, and they offered you no option to disable it.
Anyway, part of that was, even though the files were still case sensitive on the file system itself, the "correction" would automatically notice the mis-casing of the files, and correct it for you without you ever noticing. Since there was only one possible solution to the "dead link" it did it transparently without you ever noticing.
Anyway, the point of this is, Graf most likely is right, the casing simply got screwed - however, it's still possible that he uploaded them all-lowers originally and linked it with proper casing and it just worked. If that's the case, then that's why nobody's noticed any problem since March.
Another thing is, though, is that .zip's are also a case-insensetive file system, and a lot of the site was transferred using zip files. (Hey, gotta conserve bandwidth somehow) So that could also have screwed things too.