Just by reading the descriptions, it seems that there exist some third-party services (e.g., Nintendo) that will use a QR code to open a web-based login page on the device that scans the QR code (which would all be possible with no involvement from Bitwarden). At that point, you would be able to use a Bitwarden browser extension or mobile app to authenticate in the login webpage, as usual (with or without passkeys). Upon successful login, the third-party service would communicate with the original device (where the QR code originated) to confirm that authentication was successful — again, with no involvement from Bitwarden.
I’m not 100% sure, but I think that the feature request may extend to use-cases in which the third-party service is not able to complete the authentication using a method like the scheme described above.