They don’t want to ban encryption, they want to block encrypted chat apps, precisely so they don’t have to build backdoors. AFAIK it’s not possible to break signal/WhatsApp encryption without access to the targeted device, and once you have access you can get the messages directly without having to break the encryption.
I guess it could, as we have to take meta’s word for it, and a quick google search hasn’t turned up any independent security audit.