You can use SMS as a backup 2FA to login to your online Apple ID account, but that's not enough to access the iCloud keychain.
The decryption keys for that data are only stored on your iDevices. It's E2EE after all. So while you can access your Apple account via the SMS 2FA backup, you won't be able access your actual iCloud Keychain data/passkeys without some sort of access to your iDevices. (it might be sufficient if they're online somewhere and you have their login credentials?)
A bit confusing, but if it really is E2EE, then you can see why SMS alone wouldn't be enough to recover your Passkeys.
The decryption keys for that data are only stored on your iDevices. It's E2EE after all. So while you can access your Apple account via the SMS 2FA backup, you won't be able access your actual iCloud Keychain data/passkeys without some sort of access to your iDevices. (it might be sufficient if they're online somewhere and you have their login credentials?)
A bit confusing, but if it really is E2EE, then you can see why SMS alone wouldn't be enough to recover your Passkeys.