Some countries like Germany and Japan don't recognize public domain dedication. But afaik the United States does. There is one oddity in the US which is that Section 203 of the Copyright Act grants the author of a work the right to cancel "the exclusive or nonexclusive grant of a transfer or license of copyright or of any right under a copyright" up to thirty-five years after the grant or transfer occurred (only applicable to works not created "for hire"). So you could argue under that until 35 years have elapsed the work isn't truly in the public domain because the original author of the work has the statutory right to reclaim its copyright.

That said, could the author here have used Section 203 to revoke DC's license? I see section 203 requires that "Notices of termination may be served no earlier than 25 years after the execution of the grant or, if the grant covers the right of publication, no earlier than 30 years after the execution of the grant or 25 years after publication under the grant (whichever comes first)."

So maybe the author could have waited a few more years and done that?