If they were collecting fat dripping off of pigs that were roasting on the spit, then the pots were sitting in the coals.
Fat is flammable.
How’d that work?
Imo, it’s far more likely that, since this is from an era that is pre food nazi and when people actually worked really hard in manual labor on a day to day basis, fat was stored as a calorie source.
Lard is rendered from pig fat using cooking pots and fire. No, really, it's in books and everything.