The Corporate Identifier construct (which leverages serial numbers) is not exclusive to Windows Autopilot device preparation and can also be used for Windows Autopilot. Also, corporate identifiers is simply about making a device corporate or personal owned. It's not a registration/association process and is not required. For Windows Autopilot device preparation, we are working on a new method for associating a device with a tenant. I don't think we've published anything officially about this yet but it's getting close to GA (don't remember the timeline just I'll just leave it at close).
Keep in mind though that neither device registration (used for Windows Autopilot device preparation) nor this new association construct are security mechanisms or security boundaries in any way. They are simply automation enablers.