OK, I'm Confused

Cool way to make coffee, for sure, but I don't understand several things going on here...

If the point is to make the coffee in the cup hot as possible, isn't the coffee in the cup cooling just as fast as it would were it filled with boiling coffee all in one go? Also, if the sand is so hot, why isn't it too hot to hold your hand over?

Maybe this all has to do with boiling the coffee for a longer period of time, but wouldn't simmering work just as well? Or maybe this helps to keep the grounds out of the cup. I dunno. It seems the more I learn, the less I know.

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