Those black plastic plugs are inserted into the door drainage holes. I’m not sure why those are still in the door, but that is exactly where water would normally drain from the doors, through those holes.
The black plastic plugs are not solid; even if water entered into the door, it would still drain out of the hole past the black plugs, only at a slower rate. So the sloshing is not and cannot be “water in the doors”.... the sloshing is coming from the fuel tank,
If there is any debris in the door, it definitely won’t get past those black plugs... So again, I’m not sure IF those plugs should have been removed after the vehicle was through final assembly or not... Maybe someone with internal Ford assembly knowledge can answer that question...
The plugs can be removed, just grasp and pull hard either with your fingers or needle nose pliers.