But there already is a backdoor that lets them read the messages. Facebook actually implementing the functionality to take advantage of that backdoor doesn't change anything. The security of your message relies on the security of Facebook's infrastructure either way.

If Facebook implements a switch in the app that would send your decrypted messages back to them, then a malicious actor would have to compromise Facebook to obtain those message. Right now, a malicious actor can obtain your messages by compromising Facebook and implementing that switch themselves. From a security standpoint that seems identical to me.