I also often see implementations where there’s a first step where you have to select how to log in. It’s an extra click but very clear (and usually one of the options is some form of SSO where that one click fully logs you in if you already have a session open).








The logic behind it is that a smartphone-bound passkey represents two factors of authentication: what you have (the phone) and who you are (the fingerprint used to unlock the phone’s passkey store).
Anything on a PC is easily copied and can only ever be safely assumed to represent one factor: what you know (the password to unlock your password manager). Thus the benefit of getting a two-factor authentication in one convenient step falls away.
Of course it’s still super annoying, especially if you don’t really trust your smartphone OS vendor and use a portable password manager already.