An author dedicating the work to the public domain, which is the same as giving up their copyright rights, absolutely puts the work into the public domain, regardless of whether the copyright term has expired.
This time from Cornell Law:
> Once that period of time expires, or if the creator failed to comply with any legal formalities required at the time of creation or thereafter, the work enters the public domain - meaning it belongs to everyone, without restriction. The creator may also decide before the expiration of copyright to dedicate the work to the public domain, giving that new creation to the public to use.
The author cannot 'change their mind' in this case: the rights, once given up, cannot be reinstated.
There is a sharp edge here with collections of public domain works being copyrightable, but that's a separate issue, and does not affect the public domain status of the contituent works.
This time from Cornell Law:
> Once that period of time expires, or if the creator failed to comply with any legal formalities required at the time of creation or thereafter, the work enters the public domain - meaning it belongs to everyone, without restriction. The creator may also decide before the expiration of copyright to dedicate the work to the public domain, giving that new creation to the public to use.
https://guides.library.cornell.edu/copyright/publicdomain
The author cannot 'change their mind' in this case: the rights, once given up, cannot be reinstated.
There is a sharp edge here with collections of public domain works being copyrightable, but that's a separate issue, and does not affect the public domain status of the contituent works.