Well, even an instant public domain dedication is challenging in many
countries and not well defined unlike copyright because of intense lobbying
by ugly monopolists and the public domain is neglected...
The way copyright works the only person who can sue you for infringement is
the copyright holder and if they have indeed made such a statement then it
should follow they have no standing to sue you. Then again the law is a game
of chance and often the party with the deepest pockets to hire the most
brutal legal lackeys wins.
That would seem like a legit way of going about it and my non-lawyer opinion
is that it's PD now. YMMV.
However, it might be advantageous to not drop the copyright but instead use
it via copyleft thus guaranteeing a work remains free. Public domain material
is free-for-all and can be made non-free as regularly happens. Of course it
probably depends on what your licensing which option makes the most sense.
