He’s not upset that he’s being labeled as cisgender, he’s upset that the labels cisgender and transgender exist in the first place. He knows what the word means, that it’s not an insult, and that it accurately describes him. He’s acting insulted specifically to spread the idea that the word cisgender is an insult. He’s doing this to push back against our society normalizing the concepts of transgender and cisgender. If he was only upset about himself being called that, I would agree with you that we should just accept his odd preference and move one, but he’s actively working against anyone being called cisgender or transgender. That’s the problem and that’s why people are not being tolerant of his label preferences.