That's both perhaps true in a hypertechnical sense but largely false in practical substance in the US; there are a number of legal principles (notably promissory estoppel) which make the practical effect of a stated dedication to the public domain by the creator very similar to what it says on the tin, even if the creator technically retains copyright, as it becomes practically, if not technically, impossible to enforce any of the exclusive rights under copyright to restrict downstream actions like copying or creation and distribution of derivative works after such a dedication.