This is a reference to the original release date of War for the Overworld in the kickstarter which was August 2013. The game would in fact release nearly two years later in April 2015.
Rated D for Delayed is also a reference to a news post by the same title which was posted in February 2015 after the game received another short delay due to ongoing discussions with the ratings board. The title is drawn from the ESRB ratings system which typically rate their games with a letter which stands for something such as "Rated T for Teen" or "Rated M for Mature".
Essentially it's all just to poke fun at the constant delays that happened on WFTOs road to release.