Using code with copyright waiver and no license

This type of notice is known as public domain dedication. Only the copyright holder can do so.

Only the copyright owner can dedicate a work to the public domain. - https://fairuse.stanford.edu/overview/public-domain/welcome/

It is used to release a work into the public domain so that anyone can use it freely.

If, upon viewing a work, you see words such as, “This work is dedicated to the public domain,” then it is free for you to use. - https://fairuse.stanford.edu/overview/public-domain/welcome/

However a dedication must be done properly to be effective under law. It's unclear if this dedication is.

Dedicating works to the public domain is difficult if not impossible [...] Few if any jurisdictions have a process for doing so easily and reliably. - https://creativecommons.org/share-your-work/public-domain/cc0/

Additionally not all jurisdictions allow making a dedication to the public domain. So the author may not have the authority to make this dedication.

many legal systems effectively prohibit any attempt by these owners to surrender rights automatically - https://creativecommons.org/share-your-work/public-domain/cc0/

Keep in mind this dedication, if valid, only applies in the jurisdictions the Copyright holder had Copyright in, and possibly in some jurisdictions with the necessary foreign Copyright recognition, and necessary permittance of dedication to Public Domain. So, it's possible this work is not protected by Copyright in your jurisdiction. It's also possible that it is protected, and can't be dedicated to Public Domain in your jurisdiction. It's possible that it is protected, can be dedicated to Public Domain, but that this dedication doesn't meet the requirements of your jurisdiction, even if does meet requirements of the Copyright holder's jurisdiction. So there's a lot variables here making it hard to determine if this dedication is valid.

There is no such thing as an "international copyright" - https://www.stopfakes.gov/article?id=What-is-a-Copyright

most countries offer protection to foreign works under certain conditions - https://www.stopfakes.gov/article?id=Is-My-Copyright-Good-in-Other-Countries

It would be safest to contact the Copyright holder, and request they grant you permission to use the work under your desired opensource license, in case their dedication is not legally recognized.

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Note: I am not a lawyer. This answer is for education purposes only, and is not intended to constitute legal advice.

Copyright James Daniel Marrs Ritchey. This material was created for submission at 'https://opensource.stackexchange.com/questions/10116/using-code-with-copyright-waiver-and-no-license', but can also be alternatively obtained from 'https://snippetly.blogspot.com/2020/07/public-domain-dedication.html' under the terms of any of the following licenses: Ritchey Permissive License v10 (https://jamesdanielmarrsritchey.blogspot.com/2020/07/ritchey-permissive-license-v10.html), The MIT License (https://opensource.org/licenses/MIT).