Any scrolling on the main thread (i.e. driven by JS) is subject to jank, not just user-input scrolling. CSS-based smooth scrolling of anchor links can be offloaded to the compositor because the browser knows ahead of time what is going to happen, and in those cases the smooth scroll animation can run smoothly even if the main thread is bogged down.
It is a scroll hijacking library, it's just not as wretched as most. It's not animating the links but rather animating scrolling to the links by slowing the scrolling.