I'm on my pre op at the moment, some of the information i was given from my provider stated the same as yours 'plenty of vegetables except root vegetables'. However, when I had my pre op last week, I was provided with a more up to date version of the diet which stated unlimited salad vegetables or unlimited green vegetables with small portions of protein. It now also allows for 2 new potatoes with certain meals. I was also told by the dietitian that as I was generally healthy I only had to use the diet as a guide and just avoid carbs and sugars.
I know everyone's pre op diets can differ so I think the best thing you can do is contact your providers dietitian and ask the question.....I did and was better off for it!