As a lifelong American born and raised in the North but who has lived more than half his life in the South (well, in New Orleans, which is really a whole north place), I have observed that the winners (Northerners) almost always say "Civil War" while many of those who identify as Southerners prefer the term "War Between the States." These two terms are by far the most commonly used, way more than others mentioned above.

I never heard that "War of Rebellion" New Englandism, but then I've never lived northeast of New York City for more than a couple of weeks at a time.

Back to the original subject, I've only heard the one pronunciation that has already been established here as the standard one. I remember *not* knowing how to pronounce it myself, many years ago as a youngster, since I read it many times before hearing it spoken.
Coincidentaly, my wife asked me how to pronounce "Antietam" just two nights ago -- hence my intertest in reading this thread.

Kat, I think you just heard a confidently-spoken mispronunciation (not unusual) and took it seriously.