The 20 g/day of carbohydrate is an upper limit, not a target. You can certainly eat less, without ill effect. The advice about protein is to keep it reasonable. You want enough in order to avoid losing muscle and bone density, but not so much that your system has trouble dealing with excess ammonia. (That’s a fairly wide range.) The fat, which has a negligible effect on insulin, is the safest source of energy. You want enough to satisfy your hunger. Some people find protein more satiating that fat, others the reverse. You’ll have to experiment a bit, most likely, to find which category you belong to.
Note that we advise eating to satiety, not to a caloric target. The reason is that your body will adjust its energy expenditure to match the intake you give it, and if you set your calories too low, it will cut expenditure to match. This makes eating a caloric deficit in order to lose fat a moving target, and if you push too far, you risk losing muscle and bone density, because the body hangs on to fat as a reserve, well into the process of starvation. If you give your body more energy than it really needs, it will increase its energy expenditure to a great extent, but there is a limit, past which it will start storing fat again for a rainy day. Eating to satiety prevents both over- and under-eating.
BTW, that Virta graph that Eric posted is a made up example of a hypothetical 5’2" woman who embarks on a ketogenic diet. It is intended to illustrate how, by eating to satiety, your food intake automatically adjusts. At the beginning, someone with excess fat will automatically eat less by eating to satiety, which allows the body to burn off the excess fat. But as we progress, our energy intake has to rise until our excess fat is gone, and eating to satiety automatically increases, until we are getting all our energy needs from our food. Do not take those figures as guidelines for how much to eat; they are made up.