Very true metabolism is a factor in weather or not a chicken or any animal is obese or not just the same as a imbalanced diet.
Say if a chicken has a low or slow metabolism rate that chicken will be a bit more over weight than one that has a higher metabolism.
Just the same a chicken that's kept in a small/ under sized coop or is just plain lazy will also be over weigh compared to a more active one.
But if they eat to their caloric needs, activity level or amount and form of calories provided (table scraps) shouldn't matter...they still shouldn't become overweight.
My point is that this is a much more complex issue than one blanket statement can cover. And what works for one bird, may not work for another, a lot in part thanks to artificial selection/genetic manipulation (human-directed breeding). Even in wild-type birds there are variations between individuals--the aforementioned "metabolism" for example. To translate to another species, why does one dog need expensive raw human-grade food to just survive while another thrives on Old Roy? Why isn't there just one universally accepted diet that is "best"?
So, with so many variables and so many choices, how does one go about picking a diet for your flock? Some use scientific methods, some use trends (what seems to work for most), some use fads, some use "how it's always been done". You pick and then you see what it does for your flock. Are you satisfied with their performance, however you measure it? If you are, then great, you're done, issue solved. If you're not, then you have the choice to change diets completely or tweak the one you have.
So, fermenting the feed. It doesn't take any more money to do, minimal time/effort, and seems to affect performance in a positive manner. It's not the only way to feed, but a lot of people who do it are satisfied with it for their flocks. Probably not everybody...they can try a different feeding strategy or tweak this one...up to them. But, no matter what your feeding method, I do love figuring out/knowing the hows and whys behind it. It's how I'm built and why science as a field of study attracted me in the first place. If something doesn't make sense, you betcha I'm going to zero in on it and study up on it until either it does make sense, it is debunked, or a run up against the wall of not enough data/too many unanswered questions. I suppose I could just keep quiet about my findings, maybe tick a few less people off that way...but I get excited when I find out something fascinating or I figure something out and I have to share the knowledge. Again, it's how I'm built. It's also a way of gaining additional knowledge--through discussion and debate. I am perfectly willing to be wrong, but you have to prove it to me