When I had a vet visit for an RI in my Kingsnake, he had told me to soak the snake every other day to help with the RI. He also did say warm bath, not hot water. So I don't think soaking could have been bad for the snake, but the snake could have gotten some water in the lungs.

Though, since the snake was showing signs 1-2 weeks before the vet visit, it could have quickly turned into pneumonia during that time since reptiles go downhill quickly from being sick. The mucus in the lungs could have made the snake try to spit everything up but it couldn't so the snake drowned in the mucus in the lungs.

Just my thought on it, but I doubt it was pneumonia that could've killed the snake.