"He who marches out of step hears another drum." ~ Ken Kesey
It's an unauthorized and obvious derivative infringment by Harry Potter on the intellectual property of AA.
I think your reading the statement wrong. The "This image of simple geometry" is only describing the art. Its not saying why it is "ineligible for copyright". There has to be something missing about this. There are a few ways items can be considered Public domain. I would guess the art is past its copyright. Copyright is only so many years past the creation. Be it the death of the creator or so many year of use. We really need to know more about it to figure this all out. Now does it make it ok to use because its in the public domain? That's another series of questions about the use and a ton of other things. Don't forget copyright is only one way to protect a piece of art. There is also registering and trade marking. They all have their own rules and laws protecting them. My answer to Copyright is: did you create it? Did you pay for it to be created? Did you obtain the right to use it (approval or purchase -NOTE: your inability to find the creator or owner does not give you rights to use it)? If not then no you can't use it.