They can't

Microsoft makes Surface Duo. A Google certified Android device. Google forbids companies that make Android devices to make devices on Android forks.

I guess the technicality they can use here is "we're not shipping a device based on AOSP, but a software feature based on AOSP". Even Surface hardware is technically a Windows-based device with AOSP features.

Is it basically the same way how android emulator (avds) render hardware accelerated graphics?