You're correct. The same manual lists 12.5 liters in that section.

So..........I have no idea which is correct............although getting 13 quarts into the system is not going to be easy. This could also be the result of residual coolant that won't drain even with the block drain removed.

When I recently did the W-126, I blew out the lines into the heater core to ensure that this did not compromise the new coolant. I was unable to get 12 quarts into it............even though I carefully filled the head.