[djur]

It doesn't fail to meet the OSI's definition of open source. As you elsewhere conceded, the "blessing" in the SQLite source doesn't have legal weight and doesn't violate the OSD. Public domain software has always been considered open source. For instance, the Debian project, famous for their exacting standards for free software, accepts public domain software:

https://wiki.debian.org/DFSGLicenses#Public_Domain

As they mention here, it is theoretically possible that code dedicated to the public domain might still be encumbered in a way that makes it not open source: "we are unaware of a case where a jurisdiction has upheld a copyright claim to a work which has been dedicated to the public domain everywhere".