He owns the copyright for Fables. He has released the copyright for Fables into the public domain. He does not have the right to release the official comic series into the public domain because DC Comics has partial ownership of that. His contract with DC Comics is still in effect and has not been broken. He is not creating new Fables comics but is not preventing the official series from being sold.
I think fundamental to Willingham's action, and his intent, is that there now is no "official" Fables comic series. There's DC's version of Fables, but now anyone can make one with equal legitimacy.
If people treat the DC version as "official", then that has some (not all) of the same effects as claiming that Willingham gave DC the copyrights in the first place. DC can't sue anyone if they aren't the owner, but they can take advantage of fandom's desire for an "official" version to crowd out anyone else writing such a comic, and threaten competitors with lawsuits to get them to stay away (IANAL and don't know exactly how much they could do).
He owns the copyright for Fables. He has released the copyright for Fables into the public domain. He does not have the right to release the official comic series into the public domain because DC Comics has partial ownership of that. His contract with DC Comics is still in effect and has not been broken. He is not creating new Fables comics but is not preventing the official series from being sold.