The condition that will pull Mordecai out of the dungeon and away from Ruby if Donut dies.
Watching Mordecai read to Ruby in the Saferoom, Carl works through what defeat would mean for him. If Donut died, Mordecai would be done.
Carl isn't sure exactly what would happen to him with the quarantine, but he knows Mordecai would be out of the dungeon, and would never see Ruby or any of the other kids again. That certainty is what makes the scene at the table a goodbye.
The looming consequence that turns Mordecai's quiet afternoon into a last one.