I was expecting a bit more category theory. But I was glad to see that he alludes to the duality of minor chords and major chords. A simple way to look at it is like this: Pick two notes, i.e., an interval. Then pick a note in the middle of the two notes. The dual chord is the one where you swap the up and down chromatic distances. Then if you happened to pick a 5th interval, and you happened to pick the major 3rd as…
This is why I like the guitar/bass over the piano, because these relationships show themselves when you play. On a piano, if you go from C to E# the hand shape is totally different than A to C, even though the intervals are the same, so this relationship isn't intuitive.
I will say that playing multiple musical instruments gives the same kind of multiple ways of looking at a thing that multiple programming languages does. One of the things that was a little strange for me when I first picked up the flute was the idea that lower notes were further away from me as opposed to being closer the way that I was used to thinking of them with bass (particularly double bass which was my first stringed instrument). Meanwhile, on brass instruments, different pitches don't really have any spatial location at all.