That's because 1) the picture you refer to has two layers of board, one's the inner layer for the helmet, and another layer for the "face" or "mask" portion, the picture is where he's gluing the two together; and 2) there's also the inner dome ring Antman used to get the hemet shape right, which is a roughly circular flat piece of board, to which the sides of the helmet is shaped when it's being bent. He just cut out the middle of the circle once the sides are glued in place.
Go through this tutorial in a bit more detail and you'll see what he did. Good luck if you're making your own too, vode.