Bleeding Cool points out that Willingham has don't this public domain thing before with his Elementals comic book: https://bleedingcool.com/comics/bill-willingham-declares-fab... (they link to this clip where Willingham says it: https://www.youtube.com/clip/Ugkx7E0hDkYBOAdDIvVymTsdPoFnv4g... )

From this perspective it seems like what he is doing by declaring things public domain he is basically calling the bluff of the companies. Whether it is whoever claims to own Elementals or DC, he seems to be hoping that someone will take up his offer and take the companies to court for him.