What he was calling supertonic playing I would call him playing in the key of C using the Dorian mode. What do you think?
Per that old post we worked on in the past, that could very well be.....
The modes ranked by brightnessI kind of took that video as he converted all the songs to C Major to show that the second note in the key, D, was being hit a lot. But I guess that shift to Dorian would put the D as the tonic 1st note of the key.