The book discusses this a bit, actually. It basically credits the UPF theory to one Brazilian researcher, Carlos Monteiro, and discusses how he developed the theory by applying epidemiological techniques to the obesity epidemic in Brazil. It gives the specific example that, in Monteiro's research, if a family bought sugar at the grocery store, it actually reduced the likelihood that they were obese because, even though sugar is not healthy, it meant they were still cooking at home.

My understanding is that he would probably make similar predictions about your examples: people who eat fried chicken and chocolate as part of a mostly home-cooked diet are less likely to be obese than people who drink diet coke as part of a diet that mostly comes out of wrappers. He would probably also say that there's nothing inherently bad about diet coke, but there is something bad about getting all your food from people trying to make money instead of from people who love you.