Really absurdly simplistic and poor understanding of endocrinologyfat doesn't cause weight gain, a high insulin response does. a caloric surplus doesn't cause weight gain, a high insulin response does. carbs cause a high insulin response. (once your muscles are full of glucose) a high insulin response stores everything in your fat cells, both fat and sugar (this is the reason they say high fat and high carb is bad, high carbs cause the storage response (insulin), High fats are very energy dense and cause alot of fat to get stored)
From marks book: carbohydrates control insulin, insulin controls fat storage
nearly every diet on the planet calls for people to restrict calories (have a caloric deficit) and alot of them only lead to failures, technically "a caloric surplus leads to weight gain" is true, but it is not the route to weight loss. our hunger response is a very deep seated urge in our bodies and almost impossible to control, If the body "feels" like its calories is getting restricted it will enact the hunger response in full force, hense all the calorie restricting failures.
the most helpful advice to give people is to suggest they eat to control their insulin response, this involves eating low amounts of carbs (around 50 - 100 grams a day). ofcourse with carbs this low we won't be getting much calories from them so we need to eat a good amount of fats. because we are getting a good amount of fats our hunger response is quelled and we naturally eat only the amount of energy we require.
hope that helps