The author granted a license for all Creative Commons licenses, which includes CC-Zero (public domain where that's possible, the closest possible equivalent everywhere else). That's the real part that makes the rest of the section unnessesary.
a)public domain is a concept that is far from a world wide legally recognised concept (its roots are in old English law)
b) public domain scares company lawyers and it's easier to give them an actual license :D
Right, the "continental" (franco-german) IP school having moral rights which the anglo-saxon IP school doesn't have[0][1].
These moral rights tend to be perpetual, non-waivable, non-assignable and inalienable, as a result the concept of public domain in the anglo-saxon sense doesn't really make sense, instead "public domain" is a shorthand for works whose economic rights have lapsed (that is in fact the original meaning of the term as coined by Alfred de Vigny) or works which don't have associated economic rights in the first place (e.g. mathematical proofs, parliamentary transcripts) rather than an actual legal concept, and an author can only waive their economic rights through licensing.
[0] the english Crown Copyright has some similarities
[1] anglo-saxon copyright roughly matches the continental "economic rights" subset of author's rights