CC0 and WTFPL are also copyright licenses. Works available under these licenses can be used with very few restrictions (basically, as few as permitted by law), but the works are still copyrighted.
It’s simplified, but more or less accurate. CC0 is an attempt at making a public domain deed possible for jurisdictions that do not know public domain deeds like the US does. German Urheberrecht is not entirely like US copyright, some rights of the author are impossible to waive. So CC0 has a fallback for those jurisdictions, it grants a license to the fullest extend possible under the law. https://creativecommons.org/publicdomain/zero/1.0/legalcode
"Unlike the Public Domain Mark, CC0 should not be used to mark works already free of known copyright and database restrictions and in the public domain throughout the world. However, it can be used to waive copyright and database rights to the extent you may have these rights in your work under the laws of at least one jurisdiction, even if your work is free of restrictions in others. Doing so clarifies the status of your work unambiguously worldwide and facilitates reuse."
As I understand it, you can't just waive copyright on your own work - copyright is something you get automatically in most jurisdictions on the planet. Licenses like CC0 and WTFPL are operating within the copyright regime - they use the laws to make your work behave as if it was not subject to copyright - but they can't actually make your work not copyrighted.
It's like a difference between simulation and reality. CC0, WTFPL, etc. are simulating a copyright-free reality on top of the copyright system. No matter how close to perfect they get, it's still a simulation, and copyright is the underlying runtime.
It depends on jurisdiction. Some places (I think the US is one of these) do allow you to put works in the public domain, and CC0 does that where possible. The “as if” license is a fallback.