Pretty sure they were being sarcastic. I also don't find your arguments persuasive in the slightest, and I find myself being skeptical of these recent moral outcries. I'm skeptical of its sincerity, and I don't buy it. "Master" has an etmylogical background far more diverse than the dichotomy to "slave". I can wholeheartedly say that I've not once thought to make that association. It's been a title for centuries. Master blacksmith, etc. (See https://en.wikipedia.org/wiki/Master for a list)

Another example of what seems like a fake moral outcry is "blackface". And, I mean what it is being referred to now, and not the actual meaning. The racist ridicule by stereotyping ethnicity. That was "Blackface". Yet, for some reason, context doesn't matter anymore, and we end up with removing episodes of Community because someone painted their face in a cosplay of an dark elf, in exact commentary of this.

There is a significan systemic racism in the US that affects almost everything. In order to deal with those things, the very first thing would be to properly be able to identify racism. Context matters. Renaming "Master" branches is not progress. Ostracising a kid for dressing up as Michael Jackson isn't it.

Whenever I see outrage over such things I cynically think that the person is probably white, and probably doing it for attention. One thing is for sure, it only serves to detract from the real issues.