I analysed the 1500 kcal BHF diet from their Facts Not Fads booklet, and it came out as almost 170 g carbs.
Consider someone who then increases their energy intake in an attempt to maintain their weight. Proportionately, a 2000 kcal diet would have 225 g of carbohydrate.
However, if there were no carb deficiency issues on the lower energy intake diet, there is no need to increase the carb intake for the 2000 kcal intake diet. Whereas the body was (theoretically) using 500 kcal from body fat (predominantly saturated/mono-unsaturated animal fat) on the weight loss diet, ingesting the deficit as natural fat will maintain that balance.
Why is that important? Increasing the carb intake necessitates increasing the amount of insulin to cope with it. Lower insulin levels have a beneficial effect on health and longevity, while higher insulin levels predispose us to fat deposition, making appetite control fraught with difficulty (the quality of carbohydrate is also important therefore).
This also matches with our body's typical macro-nutrient usage, where twice as much energy is used in the form of fat compared to carbohydrate.