It’s the IMAX aspect ratio 1.43:1 to be precise. He just wants to fill the entire screen for the immersion. They still frame the shot as if it’s a 1.85:1 or 2.39:1, for non IMAX theaters, but in IMAX it just doesn’t show black bars on the top and bottom and you get more information and your entire peripheral vision is filled. Most of the on screen action still happens within that framing in the center.