Troubleshooting Soft and Mushy Oven Fries
I would guess that the rice flour is the culprit. The flour is absorbing the cold olive oil before the oil is able to fry it. If you were deep frying, the hot oil would crisp the flour before it was able to absorb the oil.

Whoever wrote the recipe was probably adapting a recipe for traditional fries to work in an oven and forgot to leave out the flour. Luckily rice flour is completely flavorless, so doing away with it shouldn't affect the taste at all.