I think chassis choice is just to keep it consistent. Sq3 is apparently based on 8cx gen3 which runs on 7 watts. The site you linked says sq3 has 4 medium and 4 small cores, but judging by how they run at the same frequencies as 8cx's 4 large and 4 medium cores and the benchmark scores of the two chips being pretty much the same, I think it's safe to say they're the same chip. At the very least if sq3 pulled 4x the power to produce the same result Microsoft would just use the 8cx gen3