The privacy concerns are not that Meta will read your messages (because they can’t, as you mention), but the metadata they can read such as your details and who you contact.
“So, Facebook can track who sends WhatsApp messages, when, to whom, from which location (if a user allows), etc - but not the content itself,” Rykov says “This creates a privacy concern for people who want full anonymity. These people should consider using more privacy-enhancing apps like Signal, Threema, Wire instead.”