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.
If you can move a mouse cursor by a single pixel, that would seem to be enough fidelity to scroll by a single pixel. I've never seen a touchpad that didn't meet that criteria. But then I've never used a touchpad in linux. And maybe I'm just wrong about everything. Wouldn't be the first time.
On older systems, scrolling on the trackpad would emulate mouse scroll
wheel events - scrolling my one (or more) lines per increment.
This made sense with scroll wheels, because they moved in discrete (large) clicks.
The problem isn't capturing one-pixel accuracy with the trackpad deiver, it's that at the application layer a lot of legacy mouse input APIs treat scrolling as if the user still has a 90s-era mouse with a physical scroll wheel.
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.