He's saying XBox, not XBox360. The XBox had comparable graphics to an FX5200 or FX5500, from what I saw of game screenshots on IGN. Despite that, it had a weaksauce
You're also forgetting that the CPU is important to getting things to the GPU. Microsoft and DirectX have a history of massive CPU overhead, so might have to devote an entire core of the XBox360 to graphics. OpenGL on the other hand has much less overhead - PS3 games might eventually get away with using a single SPE for graphics, leaving about 3 times more processing power available for other game features.