it's also muddy. Willingham has since clarified that his public domain dedication means that the public can't reproduce the existing comics. That's not surprising; while Willingham doesn't say so, it's vanishingly unlikely that he owns the copyrights to the artwork created by other artists (Willingham is also a talented illustrator, but collaborated with a who's-who of comics greats for Fables). He may or may not have control over trademarks, from the Fables wordmark to any trademark interests in the character designs. He certainly doesn't have control over the trademarked logos for Warner and DC that adorn the books.
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When Willingham says he's releasing Fables into the public domain, it's not clear what he's releasing – and what is his to release. In the colloquial, business sense of "IP," saying you're "releasing the IP" means something like, "Feel free to create adaptations from this." But these adaptations probably can't draw too closely on the artwork, or the logos. You can probably make novelizations of the comics. Maybe you can make new comics that use the same scripts but different art. You can probably make sequels to, or spinoffs of, the existing comics, provided you come up with your own character designs.
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So maybe Willingham should create his own bespoke license for Fables. That may be what he has to do, in fact. But boy is that a fraught business. Remember the army of top-notch lawyers who created the CC licenses? They missed a crucial bug in the first three versions of the license, and billions of works have been licensed under those earlier versions. This has enabled a mob of crooked copyleft trolls
