It's clever, but honestly I don't care how smooth it is. Scrolling should simply scroll a view up or down a page. Not invoke animation. We already have established UX patterns for playing media, slowing it down, speeding it up, randomly seeking through it.
Part of the smoothness here is that scrolling the text is 1:1 once you get down to the sections with colored headers. It demonstrates that it's possible to make a page look fancy like that without "breaking" your intuition of what scrolling "should be."
JS animations obviously don't take the place of video/audio media that you'd play/scrub through.
Hey I'm the author of the lib, exactly, I don't really "highjack" the body scroll, I'm only controlling the background animations with it, while keeping most of the body content scroll naturally with the page.