The larger hens don't usually suffer falling in the pool in summer. It happens frequently enough that i have never had an issue like this. When Elphie fell in it was late October and it was minutes and she was gone. I always say the water was Titanic cold.
I lost Dusty because she drowned so fast. So fast.
The hypothermia with Ned really surprised me but he is still a little fellow. The normal body temp for a chicken is between 105°F and 107°F (40.6° and 41.7°C). He clearly could not maintain that temperature when in 84°F water despite the sun and hot air temp of 92°F. Even 92°F is below his normal body temp.