They do seem to have experimental support for local encryption, but I don't think it's quite the win people will assume it is, since an attacker could conceivably pull the key from memory when you access Signal. A regular user isn't likely to be targeted by an attack that would retrieve the encrypted messages, and a state-level attacker can work around the encryption.

It's a hard problem to solve, and the best answer is to make sure you use hardened devices and ideally not discuss sensitive information on a handheld device in the first place.