That bolt compresses a small wedge against the bottom of the battery. Reach down there and give the wedge a tap. It may be wedged! Alternatively, more pressure on the battery. It's just sitting in a box. Perhaps the physical dimensions of the Die Hard are not exactly the same as the Optima. It may be a tight fit in the box. More power! Use a strap. Whatever you do, do not use the fender for leverage, or you'll be seeing the paintless dent remover.
Here's a view from underneath. The bolt you loosen from the top is at the bottom of the photo. You can see there is nothing except that lip on the front side holding the battery down once the wedge on the back side is loose.