Only those Ashkenazic poskim who were part of the original kitnios g’zeira (about 600-700 years ago) were permitted to determine which foods would be part of the g’zeira; we cannot do it on our own.
This is why we are safe with potato starch even though it’s more like flour than mustard seed (kitnios) yet perfectly ok. Those Ashkenazic Rabbonim never ate potatoes and, therefore, did not experience potato starch to make a g’zeira.
Another example: Rav Moshe zt”l held that peanuts were never part of the kitnios g’zeira mesorah, so he permitted peanuts on Pesach.
When I was young, peanut oil was THE oil used on Pesach by everyone. Whatever happened to that? Today no kashrus agency would put a hechsher on Pesach peanut oil. Why? Because they would be accused of all kinds of nasty things. Enough said.