I also point out that I tried to add affordances to make it easier to use and was stymied by bad browser support getting worse. I haven't updated this CSS in a while, because I haven't felt like I needed to, but the way the browsers display it in 2021 is actively worse than when I wrote it and I'm sorry about that.
I can also at the same time believe that Apple removing scrollbars altogether and not replacing them with some sort of affordance is also a UX crime, whether or not my own decisions that make are worsened because of it were deliberate.
If you're set on that, maybe you could try adding a really unmistakable indicator such as a "Scroll right for more ->" text. I'm not sure if that's good as I'm not a UX/UI designer, but probably better than nothing. Still, as a user, a would prefer something that I'm used to, such as top-down text.
The web is top-to-bottom, every user has this expectation. The usual mouse has a top-to-bottom scroll wheel, not a left-to-right one. Maybe it's not what you want, but it is what it is. You can either accept that fact and use a top-bottom layout, or accept the fact that you made a deliberate decision to create a confusing UX.