They seem to draw a line between their licenses and CC0, which they regard more as a "public domain dedication tool".
Truth is, you usually can't really throw something in the public domain even if you want to : CC0 is supposed to be a way to get as close as possible.
Being in the public domain means your work doesn't need a license, because there is no right to license. They seem to get rid of that word for that reason (it's still a license though obviously).