passkeys are not meant to be a second factor; they are meant to replace the password as a primary factor.
>to protect against the theft or compromise of your primary device.
I love Yubikeys, but the only additional protection you get by making the passkey hardware-bound is preventing an attacker who has already compromised your operating system from stealing the credential.
But! Unless you are also doing hardware binding of the session (aka cookie) after sign-in, then doing hardware binding of your credential is mostly security theater, because the attacker can just wait for you to sign in and then steal your session.
And there is standards work happening separately for hardening session security, such as DBSC: https://w3c.github.io/webappsec-dbsc/
I have no idea if Yubico is involved with DBSC, but I would hope that they are, because it would help them make Yubikeys live up to the security guarantees that I personally feel are heavily implied by their marketing.