What happens when you poor gallons of boiling hot water through used soil???
It gets wet. You may flush slats out of the soil this way, You can also do the same with cold water. 
I think they may be asking about sterilizing it. It might do the trick, the problem is there might be areas that don’t get to full boiling temperature, it might not kill all the bad stuff. If you are thorough enough it could likely work, as well as resulting in flushing much of the nutrient salts from it.