Feynman's detour is to explain to the interviewer that he can't give any satisfactory explanation beyond "the magnetic force exists and behaves like that" given that the interviewer has no advanced knowledge of physics. Trying to do so would invariably involve Feynman cheating the interviewer, causing the interviewer to believe he understood it better until he comes across the flaw in the reasoning.

It's a simple, relatively satisfactory explanation a layperson should understand, that provides a hook for finding more in-depth knowledge if the asker wants to.

The whole rest of it was unnecessary.

The idea that atoms are mostly empty space is a great analog to his rubber band example. Our attempts to explain atoms in terms of electrons cleanly orbiting a nucleus gives people the wrong picture. Atoms are not mostly empty space, electrons become standing waves when part of an atom and it's not just that we cannot measure their position with accuracy, their position does not exist. The solar system metaphor for the atom has probably caused more harm than good.