If you found this interesting, you should also checkout the Sidekick Notepad from CGP Grey / Cortex [1]. CGP Grey has talked about this before in earlier videos [2] but recently updated a video discussing new designs [3]. The idea of 'Paper Apps' and the 'Sidekick' seem to be in a similar vein.
[1] https://cottonbureau.com/p/XT9MRF/journal/sidekick-notepad