A sauna is not a passive activity. Your heart works about as hard as it does during light cardio, and that changes how to read the survival data.
Sauna gets treated as relaxation, which makes the longevity findings look like magic, but the mechanism is plain. When you sit in heat that hot, your body's main job is to get rid of the heat before your core temperature rises, and it does this by opening the blood vessels in your skin and sending a large share of your blood to the surface where heat can escape.