I think I would have to do the same, but that what I got from reading their technical paper. I hope they did not oversell themselves.
To be fair to them, the tankless water heater set up has some flaws in it. They have some types of heating coils as a measure to transfer heat. This set up is prone to mineral scaling problem. As you use the unit for a while, its efficiency would drop more or less depending on water hardness. WDRP set up, on the other hand, is an open chamber. It would be less likely to scale. Of course, it still does, but to a lesser extent.
Just to be clear, the causes of scaling come from two main reasons--1) Mineral saturation, and 2) Nucleation surface. In English, when you raise water temperature, several dissolved minerals reach their saturation and precipitate. This phenomena would be enhanced if you have more surface to build up scaling. Heating coil is perfect place for that. Thus, the engineer will have to overdesign the heating coil capacity to compensate for those down the road. However, if you could come up with a different set up that is less likely scaled. I would be convinced that your unit would be more efficient over time.
Sorry if it is a bit too technical. A bad habit of a tech guy.