If somebody lives in Zimbabwe, and they don’t like being referred to in terms of California or not California. While you’re vocabulary is consistent, when you’re speaking to this person from Zimbabwe it would be polite to not label them as a non-californian to their face.
This non-western, non-white, non-Christian, non-Californian theoretical person might get annoyed by being defined by all the things they are not. Even though every term is technically correct.
The analogy, like most, breaks down the moment we come back to the reality of the situation at hand:
1- The lines are incredibly close together. Nobody lives across the world, incredibly removed from gender. The English language itself uses gender heavily. 2- The person from Zimbabwe, in the metaphor, is going to Californian spaces and complaining that he doesn’t want to be called “non-Californian” because states aren’t real.
The context matters, and the contexts in which people use the term cisgender are almost always in direct contrast with one or more alternatives.
That said, I don’t condone harassing people, so I’m definitely against sending him messages unprompted calling him that… But he’s just in general against the concept of cisgender existing because it is predicated on the existence of alternatives, and he doesn’t believe alternatives exist.