We often think that polymeric materials are not adapted for efficient heat transfer: everything is relative.
We must keep in mind that the photovoltaic laminate is composed of multiple layers that are not good thermal conductors: glass, EVA film, back sheet (also a polymeric material). Each of these layers has a thermal resistance that accumulates to give the global thermal resistance of the photovoltaic laminate.
Behind the photovoltaic laminate, we are adding a polymeric thermal heat exchanger with a skin width of 0.7mm. Indeed the polymer is not a great thermal conductor, but it comes after the PV laminate which is even less so! In other words, the "thermal resistance" of the heat exchanger is negligible as compared to the thermal resistance of the PV laminate. And thus the heat that arrives on the backside of the PV laminate will efficiently transfer to the water in the heat exchanger, despite the fact that the heat exchanger is made of polymer.
The result is quite remarkable: the thermal power output of the DualSun Spring panel (with a polymeric heat exchanger) is only 2% less than the DualSun Wave panel with a stainless steel heat exchanger, normally considered to be a good thermal conductor!