Why fasting is needed though?
I think it's kind of the same concept as anti-inflammatory drugs.
They say to take them after eating because your stomach is full, hence the risk of getting stomach ache and other complications is reduced, because there's food and liquids that all change the "chemistry" in your stomach, slowing the absorption of the pill down.
When assuming SN, you want it to be absorbed as fast as possible, so if you fast you ensure that your stomach is empty, hence a faster absorption.