That would make sense of your losses then. They got wet AND cold and it was too much, not just the cold. It has already been down to 2-degrees here and nobody has a heater on and the 4-foot by 5-foot windows are open. I have 3 bantam Frizzles in rabbit cages outside with a tarp over the top and around all but the south facing side, and they are probably the most comfortable birds out here. So, no, cold alone is not going to harm your birds to a point of death.
Sorry for you losses though. I would cage them and get then out of that wet because that will bring you nothing but trouble.
Good luck with the rest.