Apparently this is a story/cannot be missed achievement, but didn't pop for me at first. What I ended up doing was selecting Load Game on the main menu, and loading one of the autosaves from that stage. It popped normally after that once I finished the stage.
Note: the game only keeps about 5 autosaves at any given time, so if this (or any other story related/nonmissable) doesn't unlock initially, try loading an earlier save. Otherwise, you'd need to do another PT.