My understanding is that public domain means no license, no nothing. So anyone can sell a copy of it( claiming copyright like in this case is a different story). A licensed piece is not in the public domain. As the creator you can dictate the terms of use, and you do so with a license which basically allows everyone to use the piece( or for certain use-cases depending on the license). Think of it as a hack: a licensed piece is still copyrighted, and will enter the public domain when the copyright expires( or the sun blows up, whichever comes first).
It releases into the public domain, where possible, and has a paragraph that comes as close as possible to the public domain in jurisdictions where that release isn't possible.
I think the idea in the referenced case is that a release to the public domain means that the author gives up all interest in the work.
Not to really defend Getty Images, which seems to be acting out of line.
I had thought this would be a license, not a waiver.