I'm winging it I guess, but;-
PD is PD, if something is PD, copyright does not apply, or it wouldn't be PD.
Which is why we can't host any KING, DISNEY, Marvel or DC properties here. They are still under copyright. Those companies still exist and still maintain the copyright.
If something is PD, there is nobody around that claims ownership.
Therefore it's Free Use. But anybody using these properties can't claim copyright, they can make new works, but they only own the works they create, not the rights to the character.
Amazing Man and Iron Skull were among the heroes brought back by Malibu Comics as the books were public domain. Except, in Canada, as far as I can tell, the copyright would still belong to the estates of Bill Everett and Carl Burgos respectively.
What makes you think that Centaur comics gave Copyright to any of its creators?
If they had, their estates would have sued Malibu in the states.