This claim is false. I won my battle with "excess fat" in the gym. Diet was a secondary aspect that adapted naturally as my body adapted due to the cardio work. Note that most people don't do nearly enough hard cardio though so those without the will power or having physical ailments you may be right about them. This is true for a lot of people given my observations at my gym, including many people in their 20s. I feel fortunate to have been very active sports wise my whole life especially as a kid. Some people really need to hire a coach to help them with things like running form, intervals, building a program etc.
In my case, your claim just doesn't hold up. I can pretty much eat anything I want and the main variable is the frequency, distance, and speed of my runs or rowing. Now I do eat better but you have the cause effect backwards in my situation. And I do think that for some people, obsessing about diet or fads like fasting is counterproductive and can lead to the roller coaster of losing weight and gaining it back throughout your life. My way I lost the weight, fluctuate somewhat depending on workout frequency but the initial 20 pounds has been gone permanently.
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