"However, here's a black swan to your theory. If that is true, why do people who undergo gastric bypass surgery usually achieve a more or less normal weight ?"

Who are these people exactly? I know exactly two people who achieved this - one was not really obese to begin with and lost 30 pounds. The other one just committed suicide before reaching the age of 30 (RIP my good man). Everyone else I have met or been told of never even got out of the "obese" BMI range.