Yes, that's how it worked before 1989. In that year the law was changed so that creators now obtain copyright automatically without any requirements for notice

Yes, that's how it worked before 1989. In that year the law was changed so that creators now obtain copyright automatically without any requirements for notice or registration. If the linked article is accurate and Warhol gave a disk containing the images to an engineer without a copyright notice on the disk or in the disk's contents, and without any explicit restrictions on redistribution, those images are in the public domain as well.