Overall I agree, but couldn't Edward steal the users' Facebook cookies regardless of WebAuthn? I think those last until the user clicks logout, so many of them will last forever.
Additionally, Edward can steal the cookies of every user using Facebook for that hour. But he can only steal the passwords of a small fraction of those users, because only a small fraction will start a new session; most users will use existing sessions (because the cookies last forever).
Additionally, Edward can steal the cookies of every user using Facebook for that hour. But he can only steal the passwords of a small fraction of those users, because only a small fraction will start a new session; most users will use existing sessions (because the cookies last forever).