Even then, IIRC CC0 is not that easy to apply to code (even CC do not recommend it for that: https://creativecommons.org/faq/#can-i-apply-a-creative-comm... ), WTFPL is entirely untested (even more so than GPL/MIT which are also not entirely tested in courts) and some regions do not recognize public domain. For example sqlite sells a license for what is public domain code partially for the reason of "You are using SQLite in a jurisdiction that does not recognize the public domain" and "You are using SQLite in a jurisdiction that does not recognize the right of authors to dedicate their work to the public domain".
Also, when it comes to code dedicated to public domain it is good to consider what wikipedia says about implied warranty: "The licensing process also allows authors, particularly software authors, the opportunity to explicitly deny any implied warranty that might give someone a basis for legal action against them." It is always good to include a clause saying that there is no warranty, implied or otherwise (unless of course you provide a warranty, which you probably should not do without restrictions on OSS code).
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.
Also, when it comes to code dedicated to public domain it is good to consider what wikipedia says about implied warranty: "The licensing process also allows authors, particularly software authors, the opportunity to explicitly deny any implied warranty that might give someone a basis for legal action against them." It is always good to include a clause saying that there is no warranty, implied or otherwise (unless of course you provide a warranty, which you probably should not do without restrictions on OSS code).