ok cool so when somebody logs into a site on their phone and sets up a passkey, then goes to their laptop and tried to log into the same account, how do you easily explain how to deal with this situation?

You don't. A laptop might still be a general-purpose computer under the owner's control, and we discourage those.

What I have seen is that the site gives you a QR code to scan with your phone. People are already used to needing their phone to sign in via an Authenticator app or SMS code.