According to The Wrap, a source "with knowledge of the real script assured TheWrap that the photo Bieber Instagrammed over the weekend is not authentic — and is most likely part of an upcoming Funny or Die sketch."
They added for good measure that "Bieber will not be playing Robin in Warner Bros.’ 2015 supermashup, he doesn’t have a copy of the script, has never read it, it probably doesn’t even fully exist yet, and if it did, Warner Bros. isn’t showing it to anyone."
None of this is any surprise to anyone who has been paying attention right along; the last tweet on Bieber's Twitter account--and so the one that was showing at the top of his feed--before the deceptive Instagram image was circulated was about going back to work with Funny or Die, a popular comedy site for which Bieber has done sketches before. Other aspects of the script seemed inauthentic as well, but some--including comics artist Jamal Igle, who spent a good chunk of time railing against the decision--genuinely believed that Bieber-in-the-Batcave was a real possibility for a while there.
The Wrap joins the parade of media oranizations, including ComicBook.com, who knew all along that this was almost certainly a joke on Bieber's part, but felt compelled to report on the story anyway since it had become such a phenomenon on social media and the mainstream press that it became impossible to ignore.