I was interested in the technicalities around releasing something into the public domain and found that there is precedent and case law around it already in the US:
“[T]he author or proprietor of any work made the subject of copyright by the
Copyright Law may abandon his literary property in the work before he has published it, or his copyright in it after he has done so; but he must abandon it by
some overt act which manifests his purpose to surrender his rights in the work, and
to allow the public to copy it.”[0]
Since you're on the theme of technicalities I have a slightly different technical question: How could his work ever be released, practically speaking?
Can he release it? No, according to his contract (per the OP blog post) he can only publish it through DC.
Can some other entity release it? They'd need a copy of it. But are there any copies that are actually 100% his work? If you alter a public domain work, I don't think that altered work is public domain. It's not like the GPL. I'd imagine the altered work belongs to the alter-er. Surely DC had some slight nuance, a watermark, a logo, etc on whatever copies they released. Could they file a suit against somebody who scans and re-releases an old release?
“[T]he author or proprietor of any work made the subject of copyright by the Copyright Law may abandon his literary property in the work before he has published it, or his copyright in it after he has done so; but he must abandon it by some overt act which manifests his purpose to surrender his rights in the work, and to allow the public to copy it.”[0]
[0] https://www.lawcatalog.com/media/productattach/l/j/ljp_694pu...