In the tech preview of Safari setting the authenticatorAttachment to "platform" still enables the feature. However using the similar setting in Chrome previews - the feature is only enabled as "cross-platform". Will this be eventually aligned? If so which is the correct one?
Is "authenticate with a nearby device" has the webauthn attachment "platform" or "cross-platform"?
The formal definition of the cross device sign in feature will be included in an upcoming addition the CTAP2 spec. Some details, such as the attachment, are not decided yet. Apple platforms will conform to the spec when it is finalized.