Its not, your thinking is way to simplistic. Body is not a simple machine. Hormones have dominant role here. Produce more insulin for whatever reason, you can eat whatever and you will be fat. Produce less insulin, you can eat whatever and you will still be lean. Exercise more, have more appetite.
What is unrealistic is caloric deficit, that is unsustainable, not sure why people have such a hard time understanding that. It is never about deficit on the long run.
Gut microbes are only recently being studied as a potential weight gain cause, too.
People having the exact same lifestyle and diet may have very different results.[1]
[1]: The Diet Myth from Dr Tim Spector
Not at all. This is also known for decades. Sterile mice get fat. Low dose antibiotic is used for animal fattening as dominant antibiotic source.
Okay, sure it's been studied for longer but until very recently it wasn't really in the debate—or rather, in the average Joe's mind. Go to a nutritionist and they will tell you to eat less than you consume despite it being much more complex.
So I maintain my point. In fact, most of HN, which I consider educated, probably doesn't know about that. I myself heard about it very recently.
I find HN's general knowledge extremely lacking when nutrition is in question, basically at the level of general population.
You're right that it is fundamentally about caloric deficit and my argument is that you can exercise enough that you can eat essentially as much as you want without gaining any weight.
I'm on double serving most days for breakfast and dinner since I eat 2 meals a day - and insulin wise I think I'm just normal medically speaking.