Yes John, that is likely correct and it would be a sensible design to have more insulation on the top half if there was a top half. Mine was to be mounted either on its feet horizontally on any surface, or could be mounted on a wall with the long axis vertical using the tube supplied to fit inside the outlet tube so I imagine it would have uniform thickness all over.. It is one supplied by Webasto, but not the one shown in the report. I'd say the model I have still wouldn't deliver any water over 40C after 2 hours heating and 12 hours sitting as per the test. If I drape a couple of towels over it, the temperature of the top surface of the heater under the towels is quite high indicating to me that the insulation value of the two towels is not much worse than whatever the tank is coated with. Not scientific I know, but it suggests that a few layers of foil-covered bubble wrap or similar and some aluminium tape might make a fair bit of difference.
Regardless of a couple of minor deficiencies eg the deafening fans supplied with the fan heater that three others including gait have ditched in favour of quiet ones, the hydronic system as generally described in this topic does work and makes life much more pleasant. I was hoping to take the OKA to S America so tried to make all the tanks and piping as freeze-proof as possible by installing everything inside the outer shell and where I couldn't avoid running a couple of pipes outside, they are insulated and trace-heated so when they do freeze, it won't take much to unfreeze them. AQIS antics have put me off a bit so the OKA will likely serve out its (and our) days spending at least a couple of months a year in the deserts.
Cooktop is also diesel - and that topic is a little more controversial than diesel heating.