It is not clear from the article or its online references. I suspect that it is not fermentation. It sounds like the stew is kept at a simmer. Extra water is added to prevent burnt food and is sterilized for drinking. New ingredients are added whenever they are ready and allowed to stew for hours before eating. Assuming that my understanding is correct this is the exact opposite fermentation, keeping the stew hot will kill all but the most hardy bacteria.

Almost certainly not, since the stew is simmering frequently.

Back when I worked from home, I kept a pot of stew going for months. It was pretty nice: depending on the ingredients I added, I could get a French, an English, a German, a Mexican or an Indian flavour, and when I wanted to be lazy I could just let it go down a bit further than usual.

I really don't think so. Fermentation is something that happens at room temperature and warming a stew (even as much as 40°C) would kill all the yeasts.

Also, fermentation would render the stew sour, which I don't think is a good quality for a stew.

I think that the idea of the perpetual stew is the contrary: to keep the stew always boiling on the fire in order to prevent the microorganisms from growing and spoiling it.