Alternatively, decreasing the size of the oven wouldn't make a difference. If that was the case, this recipe would work in a small toaster oven. The inverse square law is factually sound, but incorrectly applied.

My intuition would speculate the recipe works because the dutch oven is a "better" black body than the oven. So the dutch oven acts as a huge heat sink and then more efficiently retransmits by radiation that energy to the bread. As a secondary effect (or maybe this is the primary effect), the dutch oven promotes significantly more heat transfer primarily through conduction to the bread.