Just the illuminate (third faction) is missing from Helldiver’s 1, which I’m sure they’ll add in the coming months as they continue to port over content from HD1. I wouldn’t be surprised to find new content in the future that wasn’t from HD1 so I think we’ll see another faction or at the least new enemies.
I don’t own the game nor gave I played it, but a friend was telling me the game has a lot of interactable things in-world so perhaps they meant you can actually turn off the alarm in-game.