This is the point where I have to quibble.
If the area does or has historically supported feral chickens, then yes, it can indeed support chickens in that fashion without any human-produced input.
But if it does not/has never supported feral chickens then it is not reasonable to believe that it could currently do so.
Chickens living off human's waste in an urban environment and chickens living without direct feeding on a diversified farm where they have access to human's waste, spillage from other animals, spillage and gleanings from harvested fields, etc. are not being supported by the environment alone.
If it were really as easy to support free-range chickens as you seem to be claiming then feral chickens would be everywhere instead of being limited to high-productivity ecologies in warm, wet climates.
First, I think its a false comparison to allege that chickens cannot thrive free ranging on farms where they do not live feral in nature. Farm conditions are conducive for chickens in ways that has nothing to do with the farmer directly providing crops either for the chickens or for other livestock. For example, on my farm my cows eat wild woods grasses and weeds. They of course make manure from such and stir up the ground and thickets with their grazing. The chickens follow behind and pick through the manure, the bugs and tasty bits the cows stir up, and the parasitic bugs that actually bite the cows. Thus the chickens thrive because of farm activity, but its activity that has nothing to do with any effort by myself to provide food for the chickens, and the only effort I made to provide food for the cows was laying out their fence in the places I want them to graze and browse. Also, my presence and the presence of my dogs reduces predator activity around the immediate farmyard. What a bobcat may dare to attempt on a chicken 500 yards in the woods has little to do with what the bobcat will try 50 yards from my porch, as the bobcat knows myself and my dogs are a threat. The gamefowl know this to and stick to the safe zones, while factory layers wander the danger zones until they get picked off. Therefore if you want to look at history, it doesn’t make sense to look to where chickens historically live feral, but instead where chickens were historically raised free range on farms. Which was almost everywhere.
Now as to feral chickens, the reason feral chickens haven’t conquered the southeastern United States, in my opinion, is because chickens are hard wired to not disperse far. Red jungle fowl do not disperse far in the wild. Unlike wild turkeys, which will travel for miles within a single day. Feral chickens have a tendency to just live in perpetuity whereever they are released, not traveling beyond a few city blocks or a few hundred yards in the woods for many decades or even a century or more, even though surrounding habitat is conducive for spread.
Supporting free range chickens is in fact easy in farmland if the chickens are of a rustic type. They eat bugs, grass, weeds, and miscellaneous small animals. If they have plenty of those things, water, cover, and high predator alertness, they thrive. What I’m describing here is the history of the chicken. The coop is NOT the normative way chickens have been kept by humans in history. Coop keeping is an aberration that has only defined two limited periods in domestic chicken history; for a time during the Roman period and the last 100 years. Why are you resistant to that fact?