Potatoes do have carbs yes, but so do many other things like beans. They all have different effects on your BG levels.
Not all carbs are the same. Your BG levels shoot up with the easily digested carbs. Some carbs however are not easily digested, some only partly digestible and some not digestible at all. The way foods are processed and cooked affects the chemical structure so with potatoes, cooking them and turning them into a puree will make the sugars highly digestible. Simple mashing will make them easily digestible too, but not as bad. Baked potatoes are still soft and they also tend to be older potatoes, ripeness too is a factor, and personally, I would avoid them. On the other hand, I know that I am quite tolerant of boiled new potatoes. They sit in the stomach and break down slowly so they don't cause a sudden spike. Of course, it is important not to each too much because you want it all to be gone by the next meal, otherwise BG levels will creep up