Thats correct, diet coke contains citric acid. Dr pepper zero, diet pepsi and coke zero have malic acid. Some people are fine with it some will get hungry and feel bloated.
If you loss is affected or you feel hungry cut it out. Remember Cambridge say only have whats in the book. So really Its not advisable. There is new books coming out soon so watch this space on diet drink advice. May stay the same or change. x