Sugar/carbs is an obvious no no on the diet, they highlight citric acid because people would go out drinking all sorts of diet drinks, fruit teas, slices if lemon in tea or water etc and wonder why they are not losing as much weight. It does knock you out of ketosis but too much coke zero can also stall weight loss for some. As I said before - they have researched the ingredients and there is the correct balance of sugars, proteins, vitamins and dare I say citric acid. The diet works if you stick to it.
When I did W8 we were advised NOT to have any diet drinks or flavoured water even coke zero initially. Then, they said we could have coke zero as a treat but were advised not to drink too much and not include in our fluid intake. I basically still stick to these rules.
Interesting to see what Exante say!