The term "international copyright" refers to works created in other countries, which likely have different copyright laws than the United States. It doesn't refer to works created with a copyright that applies in all countries, which at the time of the creation of this guide doesn't exist.
For works created in other countries, it can be notoriously difficult to obtain information about copyright laws. This can be due to language barriers, Internet blockages, or other reasons. Several countries have recognized this issue and gathered to create the Berne Convention, which is an international agreement about certain copyright terms. If you cannot locate the copyright holder's country on the Berne Convention site, then the next step would be to contact them directly to obtain permission. If you are unable to make contact, you may want to see if you can find a similar creation from the United States, or reconsider the amount of the work that you are using (if it can be reduced).