I’m not so sure this is true. According to Apple’s own schematics of the first Apple Watch, the thickness is greater than the 10.5mm quoted here. It depends on where you measure.
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Those are marketing measurements. The watch is actually 12.46mm thick, but they measure from the top of the crystal only to the bottom of the case. They leave off the bottom sensor array for some reason. The dimension Apple uses in marketing is in the bottom right figure with all the micro measurements, taken above the cross section.
This means the Series 2 watch is actually 13.36mm. That's one of the thickest watches out there, smart or otherwise.