I bet you that if eat 5000 calories of "healthy" food and your daily requirement is 3000 you will gain weight, and if you eat 2000 calories of "bad" food you will lose weight.
You should read the parent again, more carefully. The point is not that that is not a true statement, the point is that is a vacuous statement, and therefore, the wrong question. It is not controversial that things that cause weight gain cause you to gain weight, but it's not a useful statement for either science or engineering, and we have collectively wasted an awful lot of time and research while laboring under the delusion that this was some sort of actual explanation, rather than a mere restatement of the problem.