Very impressive indeed. I dug around and found his tweet: "uses a neural network I trained on choral pieces to generate the harmonization for your melody"
Man, shoot. I compose a bit myself and am really into vocal harmony and I was hoping that he'd hard-coded some theory rules.
I did notice that it seems to vary with the past 3 notes you've moved to. If you continue repeating the same pattern of jumps among the scale, you can replicate the chords the voices slide to.
Yeah, I think I'd find it useful- I'd love to be able to ape a few of the tricks that the four voices use to slide to pleasing chords. The music theory "rules" are certainly available, but only in the same sense that the rules of calculus are available somewhere.
I did notice that it seems to vary with the past 3 notes you've moved to. If you continue repeating the same pattern of jumps among the scale, you can replicate the chords the voices slide to.