I don't think that's how it works. You can take anything you want out of the public domain, make minimal changes to it meeting a certain standard, and copyright the result, but that only gives you a copyright to the result. You don't suddenly own the thing in the public domain, which would be utterly absurd.
They may have been concerned about getting sued by someone claiming they took it from their resulting copyright work rather than the original public domain work, but even that would have been very easy to defend against and a lawsuit in bad faith to boot. (And those are just a constant threat, regardless.)
I'll copy to only being a very interested layman and not a lawyer, but I've never heard of individuals being able to just reach out and grab things out of public domain at will and fully own them. If that were possible then there would simply be no such thing as public domain, and that's not the case.