The sun can feel scorching hot while your sunburn risk stays low
The UV index is not the warm sensation of sunlight on bare skin
Sunburn comes from UV-B damaging your DNA, not from the warmth you feel. Visible and infrared light heat your skin but don't directly harm DNA. UV-B is hundreds of times less intense than near-infrared, so its warmth would be imperceptible. UV also drops off faster than infrared away from solar noon, and clouds can even amplify UV through gaps. The author uses pvlib simulations to make this intuitive.
For some reason, knowing this makes it more intuitive to me that sunlight's warmth is disconnected from the rate of sunburn.