I think the motherboard is mounted on the top with the component side facing down. Look at the 5 "buttons" below the slot of the Bluray drive versus the Xbox One S teardown...
Tommy McClain
Great find, this makes it even more plausible, from the exploded image that puts the blu ray drive in the lower part of the console as per the Dev kit which only leaves the laptop drive above which is quite thin (compared to blu ray drive) and so I think the top section will be thinner than the Dev kit and may indeed be smaller than the S.