"In-N-Out uses fresh potatoes. Sliced in the store. Then fries them."
You forgot to put a very important word at the end of your last sentence:
Then fries them ONCE.
To be edible, fries must be fried twice. Therefore, frozen parcooked french fries are superior to fresh potatoes, sliced in the store, and fried only once. That's why In-N-Out's fries are legendarily awful.