@deprecated_ii to a point you are correct. You'll have to take a guess on the intake and output of calories, and you have to just accept that there will be some margin of error.

But what is not nonsense is that you will lose weight if you limit how much you eat. You will not be able to defy thermodynamics when you eat at a deficit. It just so happens that fat people often underestimate how many calories they eat or will try to claim that their 500 calorie coffee from Starbucks "doesn't count."