IIRC, that codebase is C++ and uses QT for their GUI layer. Unfortunately, none of that can be directly applicable to AudioKit.
The piano roll is indeed a very convenient interface. I am sure it is possible to do with some custom UI code. AudioKit already has all the necessary guts for being able to play the data inside of a piano roll.
A piano roll would be a very welcome contribution to AudioKit.
A very popular minimalist sequencer that I know of myself is seq24. Perhaps you can get some design ideas regarding scope here:
http://www.filter24.org/seq24/