It seems to me that they're comparing a USB powered device to a battery powered device. The iPhone runs its GPU in a particular way, and its GPU was chosen bas

It seems to me that they're comparing a USB powered device to a battery powered device. The iPhone runs its GPU in a particular way, and its GPU was chosen based on the fact that the iPhone needs to have its battery last all day. However the Raspberry can draw a continuous 1A without rest, because it effectively has an infinite battery. So, sure, you can put a larger GPU in a device like that.