Well yes and no, they do have about 4 grams of sugar per potato but then your body turns the starch, in foods such as potatoes into glucose, a simple sugar
Starch is a complex sugar and to your body it is a sugar. Google definition here "Complex carbohydrates are made up of sugar molecules that are strung together in long, complex chains. Complex carbohydrates are found in foods such as peas, beans, whole grains, and vegetables. Both simple and complex carbohydrates are turned to glucose (blood sugar) in the body and are used as energy"