We had no idea it wasn't. According to this, this page of Japanese means that Metal Gear Solid 4 is being released on a 50Gb Blu-ray disc, as opposed to the single-layer 25Gb variety. This is significant because Kojima said a bit ago that he had to trim the game down to fit on the disc. This means the game's some kind of super-truckload of gigabytes.
Hard news. Right there.