For anyone wanting to know, the keyboard layout is that of a chromatic button accordian [0] [1].

I guess there's a C64 "executable" that he's made available but no source so I don't know what the exact keymapping is. I did find a few different resources that show the layout in action [2] [3].

[0] https://www.youtube.com/watch?v=xwsZ41pA_Vo&t=58s

[1] https://en.wikipedia.org/wiki/Chromatic_button_accordion

[2] https://okathira-dev.github.io/client-web-api-sandbox/button...

[3] https://www.rmwinslow.com/tones/