Earlier quoted context omitted.
I've dug through their minified files and from what I can tell other than dependencies they use i.e. Backbone, underscore, jQuery, bootstrap, D3, etc I believe a lot of it is custom coded. It's truly incredible.
Author here – Part 2 uses: - React for DOM rendering - D3 for visualization geometry - RxJS for handling state (i.e. turning scroll position into animated properties) - PIXI for WebGL bits (i.e. all the little circles moving around)
Ah shoulda mentioned I looked at part 1's minified code and assumed the same for part 2. Thanks for clarifying :)