No one claimed that!
Sure, yeah, but your body isn't magicking up an extra 200 calories from that 100 calories you consumed.
Let's look at your advice above:
All good advice, sure but you could easily go over your calorie limit for a day making your own fresh food, not from packets with a lot of vegetables especially if you're chucking a load of olive oil on there. Especially if you're a short arse like me. Now, 3,000 calories of fresh home-cooked meat, veg, carbs and fats are gonna be a lot better for you than 3,000 calories a day of McDonald's but you're still gonna gain weight if you're only burning off 2,500 calories a day.
Also, humans are weak and we're constantly bombarded with advertising for foods that are engineered to be tasty. We're going to give in sometimes. It's good to know "oh, theres 200 calories in this chocolate bar, I can fit that into my allowance for the day" or whatever. Are you telling me Tim Spector never has a fucking biscuit?