He says Apple stores backups but WhatsApp does not. Well no not WhatsApp itself but the backups are stored in iCloud or Google drive there. They're optional but you can never tell if a recipient has it switched on.
So IMO it's just as compromised as iMessage on this point.
I'm not sure what you call "client-side encryption". As far as I know, ICloud is not end-to-end encrypted, so Apple can read anything on your ICloud if they want to.