Basically you need to free the starch, provide enzymes to convert the starch to sugar, and ferment the sugar. In modern beer making this is done by malting (provides the enzymes), mashing (liberates the starch and converts it to sugar), and fermentation. Boiling is not necessary, it is done for other reasons (sterilizes the liquid, makes it look nicer by coagulating proteins, boiling hops adds bitterness, flavor, and aroma, etc.)
Chewing the corn introduces enzymes to break the starch down into fermentable sugars and liberates the starch. Boiling could facilitate chewing by softening the corn, while also serving to gelatinize the starch. Or perhaps they ground the corn to free the starch, boiled it for gelatinization, then spit in it to give the necessary enzymes. The yeast could be added with raw corn or other other uncooked ingredients, or just left open to see what lands in it, or with a stick used to stir the batches which would transfer yeast, or by adding a portion of already fermented corn, etc.
I don't know anything about their culture, but it seems likely to me that pots would be multipurpose. So the scorch marks on the pot might be unrelated to the fermentation marks on the inside.