I wish the Stiftung Warentest had published the full article free online. Since they haven't (at least as far as I can find) I can't comment on their conclusions.
However, there is apparently a some criticism of their test assumptions, for instance here: http://petspremium.tv/hundefutter/hu...tement-dr-berg
(Just found this via my friend Google as an example - but it makes me wish that the Stiftungs Warentest full article were available as I'd like to compare and contrast.)
The important thing is that every dog owner should educate himself/herself. Know how regulations work, which will differ slightly by EU/US/UK, etc. Know how to read a label, know what is important vs what is marketing, and most important of all - read all you can about veterinary nutrition, from various sources, speak with your vet, and evaluate the food you choose with your individual dog's needs in mind.
It's a jungle out there.