There’s a reason why it’s the last ditch protection device. It’s only used in a situation where the person is almost certainly going to die anyway, there’s a very slim chance of the fire passing the zone fast enough for the shelter to accomplish anything. It’ll will protect them from being in the center of the fire for a few minutes tops, if the fire hangs out in their area too long, or burns too hot forget it. It’s a slim chance off survival device used in situations with no chance of survival.
When you get your Red Card from the Forest Service to work the fires, these survival shelters are called “Shake and Bake” - it is widely accepted that your chances of survival with them are small indeed.
Prayers to the fallen.