There's a lot of back and forth on the discussion, and summaries are included in the tops of almost all of the linked articles. In general, the name "master" branch was commonly used in proprietary git predecessor BitKeeper which almost all of the early git developers were familiar with [1] and it did use master in a master/slave meaning. (It had a concept of "slave repositories".) While git has never had a concept of "slave branches" and the meaning may very well have meant to evoke one of the other etymologies, the word was "in the air" because of BitKeeper, which did intend it in a master/slave relationship, so git is guilty by association if not guilty by intent. Which is fine, no one is suggesting git is guilty by intent, they are simply suggesting it is time now to do what early git developers failed to do and question the association (both associations, using the word simply because it was a word commonly thrown around in tech up to that point, and in the association of master with chattel slavery).

[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.