Horizontal scrolling seems to work just fine (macOS with Magic Mouse; same with trackpad). The fact that they included a workaround for those who don't have input devices with horizontal scrolling is a nice touch, yet it didn't interfere at all with my horizontal scrolling.
But that’s just the thing, and why scrolljacking is so bad—I’m on a platform with excellent horizontal scrolling facilities, and it’s ruining that. Based on my experience in this space, but not their code which I haven’t seen, I guess that this will affect at least all Windows users with precise touchpad scrolling. From my perspective, it’s a terrible trade-off. User agents need to handle this better, urgently.
I’m saying that at a minimum I expect all Windows users to be affected by this.
It’s not a Microsoft problem at all: it’s that the web platform simply doesn’t support getting this type of scrolljacking right. It’s just not possible due to how scrolling is handled in the browser, how wheel events work, and the platform’s propensity to asynchrony (so that not only is it not supported, it can never be supported). The best you can achieve is to detect OS and browser and guess at how it’s going to behave, and emulate its functionality. And quite apart from how such things change over time, that’s not guaranteed to work at any particular time: such configuration is flexible per OS, and hardware (which you can’t detect reliably) can influences how it will do things as well. Inertia is a particularly good example of that: it varies in all of these ways, by hardware, by OS, by browser and by configuration in all three of these places. Now I don’t have macOS handy, so I can’t see how it handles the scrolling there. Perhaps that site’s developers (or some library they use) has done that emulation and tuned it for macOS, but not elsewhere.
I'd say this is a decent tradeoff.