Well, the first thing you need to know is that, as the title of the submission clearly states, this is about touchpads. Not keyboards. So, y'know, pointing out that the down arrow isn't super relevant isn't exactly coming out of nowhere.

But assuming that their version of smooth scrolling does, in fact, work the same as Apple's, it's not even a matter of "it smoothly animates scrolling down by one line;" it's that you can scroll by individual pixels, rather than by lines, using the touchpad. I suspect that a certain amount of work also has to go into ensuring that the scroll animation is both smooth and well-synced with the user's finger motion on the touchpad, but I've never done work that low-level, so I'll have to defer to anyone with better expertise there.