I found a comment in an old repost on Minecraft Reddit of the first appearance of the Enderman, we can read a comment from the user Celsius1414. July 26th, 2011.
Is this the inspiration for disc 11?
From what I researched, most of the game's discs were added in beta 1.9 pre-release 2 around September 29, 2011. So we could say that the events of disc 11 is an encounter with the Enderman based on a Reddit comment.