Not to be a wet blanket, but such an image is easy to fake, and even if it is real it may not be correct. It could be a case of a careless (or overworked) rep sending out a form letter meant to address questions about batmobiles the game shipped with--as opposed to the upcoming DLC.
The fact that nothing was mentioned in Arkham Insider makes me skeptical.
After the downloading the latest update which includes the prototype Batmobile, the game displays an "all Batmobiles are useable in Story Mode" message.