Sorry guys, you are correct. For some reason I had it in my mind that power came in through UVCC on the 16U2, was regulated there and came out of it's VCC port to supply the 5V line. Seems silly, but this is the only thing that made sense to me, because the 16U2 is the only thing I saw USBVCC connected to. Now I see that it's connected to the MOSFET down below as well...