I haven't dug into the docs yet, but I don't quite follow this. How does it know that the ex-iPhone owner and current Android owner are the same person (and should have access to the same account)?
If you did switch ecosystems, you'd probably need to do the whole re-enroll dance on every website that had your previous key. I don't see any sort of passkey backup/import functionality coming anytime soon simply because people don't change ecosystems that often, and those that do can simply keep both their iCloud and Android-based passkeys on services.
Passkey works on non-Apple devices by showing a QR code on that device, having the iPhone scan it, then a BLE connection being established to allow the phone to remotely sign webauthn requests. I only mention ‘switching ecosystems’ in the sense of moving between Apple and Android mobile devices.