That alone may be enough to offend Person B. They feel Person A may be creating an uncomfortable environment, intentional or not. Person C may not even be actually offended, but other people of Person C's {race,sex,etc} have been offended by similar statements. How people apply judgment is an individual process.
Should we care about how any of these people feel? Is Person {A,B,C} right or wrong to feel how they do? We can't control how other people feel about things, but we can try our best as individuals to promote welcoming environments, but recognize we are human and won't always get it right.
Master vs Main is not a big deal. Changing the default without any technical advantage is a pain in the ass. Had they bundled it with Git V3 when moving to sha256 and everyone having to rebuild their repositories anyway (another mediocre technical idea), it would be palatable.
I think what's often missed is that this creates an unwelcoming environment for minorities who don't share your ideology.