We are not sending a rocket to the moon here. Just because something cannot be measured exactly doesn't mean it's worthless to measure.
If this were the case, Weight Watchers (which is at its core a calorie limiting system) should have a near 100% success rate. It doesn't. A casual perusal of Google shows closer to a 50% success rate for a 5% body weight change.
I can't think of many medical treatments where we go "yeah, a coin toss level of success is good enough for me."
As for the accuracy of the measurements - it does matter. Healthy weight loss rates are, mathematically, in the 10% calorie reduction range. That's less than the error rate for the caloric labeling of many foods.
First off - that's an astonishing success rate against a backdrop of an article saying, literally, surgery is the only reliable option.
Second off - You're assuming 100% compliance. I know people with personal trainers who are gaining weight because they don't stick to the program. I halfheartedly went through a weightlifting routine for years and got nowhere because I never really stuck to the program. 100% compliance in the field of weight loss is just not going to happen.