Maybe it is not listed by the OSI, but it is 'open source' in practice, if the source is also in the public domain. The FSF's definition of free software makes more sense not only ideologically, but also because it does not try to be exhaustive.

One problem is that a large part of Europe doesn't understand the concept of public domain. I'm not legally allowed to not own my creation, except by transferring it to someone else. That's why licences like CC0 exist.