Strictly speaking, I believe that it's Creative Commons rather than public domain. Pretty similar, but IIRC it addresses some differences in European law.
I can't speak to public domain or not but it is available for download. If Wikipedia had a problem with it being downloaded, I'm sure they would be asking Google to remove the Kiwix app from Google Play.
Commenter admits that it isn't public domain. So Wikipedia isn't a great suggestion for this answer, although I would argue that it is indeed "very valuable or worth archiving"
Wikipedia is under the Creative Commons Attribution Share-Alike License which again, isn't public domain. However it is a largely permissible license which allows people to do close to whatever they'd like with the material - which some people might find similarity with the public domain if they don't know how the licensing works.