The arty assumes that open source means share-alike, ignoring the existence of open-source licenses like BSD, Apache, &c, which do not contradict article 5.
As undoubtedly other people have mentioned: they did not ban all open source. They banned GPLv3 and related licenses. (although it's not mentioned in TFA, I think AGPL also is not allowed. Would make sense, since I can just relicense any code under the AGPL.)