https://opensource.org/license/unlicense
An advisory blog post warning people not to assume that "public domain" code is actually unencumbered is not the same as saying that actually public domain code is not open source.
> It is an attempt to dedicate a work to the public domain (which, taken alone, would not be approved as an open source license) but it also has wording commonly used for license grants.
https://lists.opensource.org/pipermail/license-review_lists....
It's clear OSI considers this extra wording, above and beyond the public domain declaration, to be what qualifies it as truly unencumbered. SQLite's license does not contain similar language, and has not been similarly qualified by the OSI.