did not actually file copyrights on their art with the U.S. Copyright Office.
The way they’ve worded this isn’t really a sufficient explanation of how this works. An artist is automatically granted copyright upon the creation of a work, so it’s not that they don’t have the right to protect their work. It’s just that, without registration, you cannot file a lawsuit to protect your work.
Copyright exists from the moment the work is created. You will have to register, however, if you wish to bring a lawsuit for infringement of a U.S. work.