I disagree, these unix sockets are user interfaces, albeit not graphical user interfaces. I am a user who's trying to interface with these players, what could these be other than user interfaces?

And unfortunately 'lack of documentation' is putting it lightly. VLC's barebones manpage directs users to https://www.videolan.org/doc/, which in turn says the documentation doesn't exist and gives the user another link to a wiki. If you dig around on the wiki you can eventually find this brief acknowledgement of the interface in question: https://wiki.videolan.org/Documentation:Alternative_Interfac...

Documentation is part of the user experience (who is documentation for, if not for the users?), and in this regard VLC is very poor. I think this relates to VLC treating technically inclined users like second or third class users.