The physics is the same in either case, In the fridge you are trying to reflect heat away from the fridge and back in to the boat envelope. The tri-ISO still has insulating layers bewteen the reflective ones. I suspect that the foil layers are mylar which has one of the lowest thermal conductivities of any material at 0.0001 W/mK, while the intermediate separating layers will have a more typical 0.02-0.04 W/mK thermal conductivity. Triso gets a thermal conductivity of 0.0057W/mK, so around 4 times better than Celotex/extruded polyurethane.

Products such as Celotex will fall in the 0.03 range, and draws its insulating benefits from additional thickness.