By this reasoning, abstinence-only sex education is a sure way to stop teen pregnancies and spread of STDs.
If you opt for it, then yes of course, abstinence-only sex education is a sure way to stop ten pregnancy.
Similarly, if you opt to change your lifestyle and start living healthier, and follow through, it will work.
If someone tries to force you, of course it will fail (in both of these cases). There is no way to stop someone else from being fat. We can only change ourselves.
Being fit and maintaining the normal weight is not the same thing as reaching a BMI of 50 kg/m² then losing it. I've seen fitness influencers take up that challenge only to find that "changing oneself" is significantly harder than it used to be when they were fit.