‘Free software’ and ‘open source software’ (as respectively defined by the FSF [1] and the OSI [2], which is how they’re usually used in practice) have overlapping definitions. The project in question is released into the public domain via the Unlicense, which qualifies as a free software ‘licence’. Many of the other projects use the MIT/Expat licence, which also qualifies as a free software licence.
[1] https://www.gnu.org/philosophy/free-sw.html [2] https://opensource.org/osd
If anyone is curious on FSF's comments about various licenses: https://www.gnu.org/licenses/license-list.en.html
I also use Unlicense. It's literally the most permissive license you can have lol
The caveat with the Unlicense is that it doesn't work in some jurisdictions, and the work may be considered literally unlicensed, as in nobody except the copyright owner can use it. In practical terms, of course, I doubt anyone using the Unlicense plans to come after you for copyright infringement, but it's something to keep in mind. This is why many organizations recommend instead using something like CC0, MIT etc.
And how exactly does it not qualify as an open source license? Seems to meet the definition as far as I can see.
No claim was made that it is not open source. The contention was over if it was a free license or not:
> not free software
which it is. As F3nd0 said, it's both.