If anyone comes across as though they are the keto police, feel free to ask them for the data on which they base their advice.
There are no rules for keto, beyond eating as little carbohydrate as we can manage, but a whole food diet is to be preferred, for a number of reasons, if one can afford it. That said, Dr. Eric Westman’s obesity clinic had one particular success story, a homeless man who lost an enormous amount of fat by eating at MacDonald’s, not ordering fries and throwing away the burger buns.
We recommend eating less than 20 g/day of carbohydrate, not because 20 g is a magic amount, but because it’s a level of intake that guarantees to get almost everyone into ketosis. Your tolerance may be higher than that (it may also be lower, if you are particularly insulin-resistant). Newcomers don’t always want to get into the underlying science, but rest assured that there is valid endocrinology supporting our recommendations.
The point of a ketogenic diet is to keep the insulin response to the food we eat as low as we can for as much of the day as we can. We benefit from both the lowered blood glucose (carbohydrates are all just glucose molecules arranged in different ways) and from the lowered blood insulin (while a certain amount of insulin is required for survival, chronically elevated insulin is just as damaging as chronically elevated blood glucose). The ketogenic diet therefore has as its primary effect the restoration of metabolic health. The loss of excess stored fat is a consequence of the lowered insulin levels and therefore just a pleasant side effect, when you get right down to it. (Insulin is one of the major hormones involved in getting the body to store fat.)