Snyder shot the movie in 4:3 because of the movement of the characters. Most films have travel going horizontally, right to left or vice versa. He felt that in this movie the movement is more vertical, with the flying & height of Steppenwolf.
His story is that he wanted the movie shot in IMAX format, which is really close to 4:3. Unfortunately, if you look at the cut, it's just not framed that way. There is way more empty space above the character's heads than in the theatrical version. He might have rationalized that story after the fact, but the film was clearly not shot that way intentionally.