Thank you so much for the reply!
It does look like the test sites actually are not even checking for the userHandle at all as I can pass in a garbage string and it will still work (All the other fields will specifically fail if I mess them up). So this looks like the issue for sure.
What I'm still confused on though, is how do I get that "user Id"? In the passkeyRegistration the only information I have access to is the
ASPasskeyCredentialRequest.credentialIdentity.user