I think that linguists are the least surprised people in the world to observe the coining of the word cisgender.
Once transgender became accepted as the word for describing a person whose gender doesn’t match their sex, an opposite word was bound to develop. That’s just how language works.
The fact that the Latin prefix trans had already been widely adopted made cis all but inevitable.
Anyone who studies language much is probably already familiar with the trans/cis prefix pair. They’re used with plenty of other words and have been since long before Julius Caesar was a spark in his great grandfather’s eye.
How many Latin students over the centuries have moaned over the difference between transalpine and cisalpine Gaul?
Good explainer article! Thanks!