I’m surprised people think this is odd since the original Steam Controller was the same - it’s a Steam Input device, not XInput.
If you consider what it was designed for, it makes sense. This isn’t another generic controller but a controller designed for a Linux/PC-based video games console (Steam Machine).
If you boot into a desktop UI without Steam running, desktop UIs don’t support xinput devices to navigate around them.
The Steam Controller thus defaults to presenting itself as a keyboard and mouse so that the UI can be navigated without Steam running.
If it was xinput, you’d be reaching for a keyboard and mouse to plug in just to click Steam and then immediately no longer need them.
That’s why it’s not an xinput device.