I started a game up in this for the first time in a few years. I don't ever remember giving the old guy in the Undercity his journals and the whole Outcast group leaving their camp. Maybe I forgot about it or never did it before. Either way, is there ever any answer what happens to them?
Hard to say, but I think they actually make it. Your group clears out the gangs and the monsters down in the tunnels paving a clean pass for them. The sith bombardment annihilates the surface of the planet but the undercity seems to be well insulated...much like a bomb shelter way below the surface. Although I did not read anything official, it is safe to assume that most (maybe all) reached their destination.
I assumed they survived, since the Promised Land was a self contained colony deep underground. Its likely the Sith bombardment didnt penetrate as far as the Undercity, let alone the areas below that.