Actually your article shows I am right. The GPUs are soldered onto the motherboard it is just that the motherboard is split into three to wrap it around the triangular heatsink. The boards with the GPUs on them are not just GPU boars e.g. one contains the SSD port too and so are functionally parts of the motherboard. However if we ignore semantics and just look at it pragmatically nobody, not even Apple, has ever produced an upgraded version of these boards so whatever you think about the motherboard, the GPU is clearly not upgradable.