What the article doesn't mention is that the "extra reserves" in the body, i.e., the calories that are currently used and are not burnt when exercising, must be

What the article doesn't mention is that the "extra reserves" in the body, i.e., the calories that are currently used and are not burnt when exercising, must be a constant (the body can only skip burning calories that it gets) whereas exercise can be scaled. Say you are overweight on a 3000kcal diet and start exercising. It is physically impossible to not lose weight if you exercise for more than 3000kcal. You might not lose weight if you exercise a more modest 1000kcal or even just 500 - but the statement that you cannot outrun your diet is trivially false. It might just be much harder than one might naively assume.