A passkey does not contain and is not derived from biometric data, so one cannot login to an account using biometric data alone.
If one wanted to use biometric data to access a Google Account secured with a passkey, one would:
1. Need to find a device with that passkey on it (or an account like iCloud Keychain or 1Password that contains the synced passkey). Biometric data could be used to unlock the iPhone, in theory. I'm not aware of this being done in practice.
2. Then unlock that passkey. On iOS, biometric data could be used to perform this step, just as accessing the iPhone in step 1.
If you hold the power/volume buttons or do a Find My lock, it disables biometric auth on an iPhone. I assume there are equivalent tools on Android.
So, if I lose my iPhone and someone also scanned my face, they could login to my Google account by generating a face accurate enough to fool Face ID, and only if they did it before I marked the phone as lost.