zahavasdad,
Tapioca is not Kitniyot
Correct. But its flour is often used as a substitute for wheat flours in gluten free recipes. So if you want to prove that potato starch can produce bread as a standalone, you should find a recipe that doesn’t use other flours as a crutch.
But at the end of the day, the ability for potato starch to produce fluffy Pesach cakes isn’t the issue.