That is Moxie's main point, full disk encryption should be used everywhere, and encrypting Signal's message database on Android is purely due to full disk encryption on Android being a steaming pile or bricking your phone.

I should, and do, but a lot of non-techies don't, and this design decision might end up with their Signal messages compromised.

I mean isn't one of Signal's target audience people like journalists and activists?

Yes, but the point is, if those people don't use ecryption, then they have a problem anyway - and if they do, but their system is compromised, than the attacker gets anything anyway. In either case, I think Signal's job is to secure the messages going out and coming in and not protecting the disk.