Public Domain Publishing sure sounds like everything it publishes would be in the public domain, but after looking on many pertinent pages, and reading the one you linked to in its entirety, I see no explicit statement that the articles published there have been placed in the public domain by their author.

Everything published, and even some things that aren't, are copyrighted to the author unless the author has explicitly put them in the public domain or the copyright on the work has expired. Neither of these appears to be the case here.

I think that the name "Public Domain Press" may be intended as a public relations device rather than a statement on the legal status of the articles published by it. The continued emphasis on money and editorial control on the web page tends to support this conclusion.