I personally think its just crap engineering. You wonder how its still possible in this day, but BMW managed to make real clangers like the N20 engine so i wouldn't put it past them.
I've felt my main Rad on warmup - the hot water slowly bleads the full way across it while the engine idles away with the thermostat closed, this would be worsened by driving with a headwind.
The Thermostat is downstream of the Gearbox heat exchanger AND the Rad so there's a huge amount of thermal conduction going on away from the engine block. Most manufacturers put the Thermostat directly downstream of the block and certainly upstream of the Rad.
I also wonder whether ~10% coolant temp could contribute a whopping 20%+ to fuel economy. I certainly doubt that's the cause. The engine should be in closed-loop emissions control/fuel delivery well before that.