Earlier quoted context omitted.
It's the ubiquitous "master" that everyone is trying to get rid of. There are details in several of the linked articles. TL/DR: The master/slave analogy is problematic and lots of projects have decided we don't need that terminology around any more, when we can pick less problematic names easily (easier now that there is a config option for it without needing to create a full git init template).
That’s not the type of master that’s it’s referring to. But I’m not sure etymology really matters I guess.
[1] Bitkeeper was the SCS that the Linux kernel had been using up to that point, and whose dropping of free licenses to open source projects like the Linux kernel was the immediate cause of the creation of both git and Mercurial.