Realistically, I have lost close to 10 lbs recently eating a diet of Boston Creme donuts, coffee, and bourbon. My cholesterol in October was 177, and my blood work was otherwise perfect from a physician's perspective.
I was strict Paleo for well over a year. I am not arguing that my recent donut diet is a good choice for anyone, but I feel better, lift better, and look better when I am not eating strict Paleo. Fact.
So, if someone comes to my gym, fat, unhealthy, and unhappy, then yes. Cutting sugar and processed foods and focusing on eating meat, fruits and veggies is a standard recommendation.
But, in terms of weight loss, calories in/calories out still works, and genetics are, um, real. Take a look around McDonald's. There are a lot of skinny people eating lunch there every day. Skinny doesn't always equal healthy, and a healthy diet doesn't mean you won't get cancer or die in a car accident.