It’s also voluntarism: the OS, as it is entirely maintained by willing members of the community.

Honestly, probably moreso in most cases, as while socialism is fine with getting funding from taxation voluntarism sees that as coercive by nature, advocating for voluntary donations (time/money/labor) to help out the community instead. Some stuff was developed with tax funding for sure, and some directly by the NSA like SELinux iirc, but I think it’s still mainly private citizens/corpo donations and volunteers/people that get paid using those donations.