Look at the name people give it. Scroll-hijacking. The web designer hijacked the scroll action and modified the behavior to something other than the os and/or browser level setting. When I scroll, I expect to scroll based on the settings in my browser. I don't want your website to stop scrolling and use my scroll action to play through the frames of your slow-loading animation. I just want to scroll down and keep reading.

It's not the appearance so much as the behavior. For example, I have (probably like many) an intimate muscle memory associated with the sensitivity of my touch pad for scrolling. The deviation from how I expect the software to behave using the same physical interactions I normally use suddenly makes me feel like a clumsy oaf.

I have a touchscreen laptop, so the inertial scrolling often gets broken when sites scrolljack. That said, styling the default scrollbar (except in Firefox, where it can't be done) is a different behaviour, and whilst imho it should be discouraged for "regular" sites like this example, is totally fine when scrolling small widgets.

There are some (very few, rare) scroll-hijackers that work pretty well. This one is not one of them. In fact, the one used on this site is one of the worst. Why I say this is its behavior. Instead of smooth scrolls, it seems like there is a certain threshold at which the scroll bar "ticks." They mask this tick by slapping on it a tween animation. So it's a bunch of ticks that result in tween animations. There are beautiful scroll-hijackers that use complicated math to make it an actually smooth experience. This one is neither smooth nor helpful. There are also those scroll-hijackers that are employed to do card/slide/page based interfaces. Those are still awful, much more understandable than this garbage.

I hate it because it completely distorts my view of how the input works. It's just as annoying as changing the velocity of the mouse on a page. Don't mess with inputs.