This seems like the standard to allow multiple passkeys, which makes sense... the passkeys are device specific, (not considering synced ones like bitwarden)... So if i login on my windows pc with a passkey i also want to be able to login on my iphone with a passkey, if they didn't support multiple passkeys per account this wouldn't be possible, even without taking into account different people in a family having a passkey.
Passkeys are very much intended to be synced by various WebAuthN stakeholders, for better or worse.
But yes, any site that e.g. wants to support roaming authenticators ("Yubikeys") needs to support multiple credentials, or recovery from a device loss gets very tricky.
