This is why the Creative Commons organization created CC0 in the first place to get around the differing and/or vague implementations of "public domain" [0].
As the author is most likely American and talking about Americans, I think my critique still mostly holds but you're absolutely right that once we go internationally it gets much more complex.
I only know of the Berne Convention [1] but doing some superficial research, it looks like their "fair use" doctrines are overly vague [2].