macOS is a whole thing in terms of smooth scrolling, or HID (human interface devices) in general. It seems like Apple just doesn't put a lot of effort in terms of working with third-party HIDs:

* There's a standard way to enable high-resolution scroll reporting (pixel-level instead of line-level), but Apple doesn't use it.

* There's a standard approach to multi-touch digitizers/trackpads (documented and I think to some extent created by Microsoft, called PTP) which Apple doesn't support.

* Apple's own Magic Trackpad speaks a proprietary protocol and it appears you can only speak it if you claim to use their USB VID/PID. And I don't think doing that would go over well in a commercial product. (And if you do manage to speak it, it turns out their driver really doesn't do two-finger scrolling well with tiny trackpads anyway. They probably only tested it on the generous dimensions of their hardware.) (Also, it attaches to your entire device, so having an additional interface with a different driver doesn't appear to work either.)

But...you can inject smooth scrolling events via Core Graphics. So you can run a userspace program with accessibility permission that scrolls smoothly. And you can also communicate with USB devices from such a program. There are some existing programs for doing smooth scrolling with standard mice (Mac Mouse Fix is one). I'm writing a userspace driver for PTP to make my keyboard's built-in trackpad work properly.