Frozen diced potatoes are partially cooked but require additional cooking before they are ready to eat.
You should cook the frozen diced potatoes while they are frozen still because if you let the frozen diced potatoes thaw out first then they will turn soggy and brown and won't be as good.
So cook them directly after taking them out of the freezer.
It takes only 5 or 10 minutes to cook the frozen diced potatoes and get them to where they will be good to eat.
Most store bought commercially made potatoes such as diced potatoes, hash browns etc are usually already cooked at least partially or require just heating them up.
But when you put the frozen diced potatoes in your dish and bake them or cook them on the stove you'll be heating them up anyway which helps to cook them.