MacOS 9 doesn't know what to do with this thing.
It sees it as a USB HID and attaches the generic HID driver, which does nothing.
By making it a boot keyboard, I am able to get the HIDKeyboardDriver to attach to it, but it still doesn't register keystrokes.
I suspect this has something to do with the CircuitPython HID keyboard class implementing both IN and OUT endpoints; every keyboard I've tried that works with MacOS 9 has only an IN endpoint.
Next step would be to set aside the entire firmware and try a minimal CircuitPython USB HID example that builds a keyboard manually from the documented Report Descriptor and sends a repeating test pattern or something, to see what is required to get keystrokes to register.