This is a bit misleading, IMHO. The important difference between CC0 and public domain is that PD is a copyright status, while CC0 is a license. Which means you use CC0 to grant PD-like status to works you hold the copyright for. You cannot (should not) attach a CC0 to somebody else's work just because you think it is PD. And vice versa: while stating "I release my work into PD" is probably usable in the US, it is really incompatible with the continental construction of copyright; for that, the explicit granting of a "most possibly free" license is a better fit. (Even though you might doubt even CC0 is bulletproof there...)