I don't think this is what's going on here but CC0 was expressly created because not every jurisdiction had a concept of "public domain", so a special license was needed to make sure it acted as if it were in the public domain for those cases.
From a CC0 FAQ [0]:
> Do I have to attribute the person who applied CC0 to their work?
> No, there is no legal requirement that you attribute the affirmer ...
From a Berlin Universities Publishing FAQ [1]:
> ... Since a waiver of copyright protection is not possible under German copyright law, CC0 is equivalent to a waiver of all possible rights and legal claims by the creator.
But, regardless, public domain or CC0 doesn't need attribution whether it's in the US or Germany.
[0] https://wiki.creativecommons.org/wiki/CC0_FAQ#What_is_the_di...
[1] https://www.berlin-universities-publishing.de/en/beratung/li...