Nothing wrong with your math.
Your knowledge of human physiology, on the other hand....
Sure, for some people it's that simple. For others its like telling an alcoholic to just stop drinking, or a smoker to just stop smoking, or a meth addict to just stop taking the stuff.
Yeah, some people, maybe most, can do that. Some people can even shoot heroin without getting addicted to it. Probably most people could lose weight if they were just willing to focus on it. (I'm down 55-60 lb from my peak, from technically obese to normal. Halfway into that a friend asked me how. I told her "just stay hungry".) Some can't, not without causing severe damage in other ways.
And a few years ago, doctors were saying you don't have ulcers because of bacteria in your gut, you have them from eating acidic/spicy food.