I would be interested to know who they polled and how the question was asked. I would include Stalingrad in my list.
I doubt if the magazine polled anyone. Being a Civil War magazine they were aiming at people who would read a Civil War magazine. If it was a World War Two magazine, I expect they would use at least one World War Two battle.
I would think I studied the Battle of Waterloo very briefly, Gettysburg a bit more, but not sure any high school history class I was in even touched on the Battle of Marathon. Schools differ and your high school may have studied Marathon.