According to the book, the two students treat the relevant case alike with or without fi, while Abu Hanifa distinguishes the two formulations. The distinction depends on how strongly fi makes the coming day a temporal container.
«أنت طالق غدا»
You are divorced tomorrow.
«أنت طالق في غد»
You are divorced within tomorrow.
In the second formulation, the text reports that Abu Hanifa recognizes an intention referring to the end of the day; it does not report the same opening for the first wording. The lesson is about interpretive consequences of a small particle.