Completely agreed about the nonsensical first claim. We have many third-party clients for other messaging platforms where privacy and security are a primary feature. It's completely tenable, especially for a player like Apple.
Or put another way: If the privacy and security of imessage is compromised by someone building another client, I'd argue that you never had either to begin with.
Or put another way: If the privacy and security of imessage is compromised by someone building another client, I'd argue that you never had either to begin with.