Isn't the phone filesystem encrypted?

Depends on quite a few other factors, but if someone with a GrayKey or Cellebrite appliance gets your phone, there's a good chance they can get in both in BFU and AFU states, even if locked. Once unlocked (or broken into), stock Signal offers you zero protection, while Molly forces them to start a brute force attack against the password you gave Molly.

This is less true for fully patched GrapheneOS devices than it is for fully patched iOS and other Android devices, but this space is basically a constantly evolving cat and mouse game. We don't get a press release when GrayKey or Cellebrite develop a new zero day, so defense in depth can be helpful even for hardened platforms like GOS.