It still wouldn't be a security problem, since WebAuthN includes the hash of the visited domain in the signature.
So even if Bitwarden would go blatantly out of spec and allow usage of a passkey created on and scoped to a.com on b.com, the assertion signature would effectively say "I want to login to b.com", which a.com would simply reject.
That's what makes it so much harder to phish than auto-filled passwords (which could still be MITMed e.g. through usage of attacker-installed TLS certificates).