This might be daft question but I'll ask it anyway, how were they able to do this? Is the official version opensource or did they have have to reverse engineer the whole thing?
http://minecraft.gamepedia.com/Programs_and_editors/Mod_Code...
The protocol is also well documented: http://wiki.vg/Protocol
Aside: Documentation obtained this way is available at http://wiki.vg/Protocol.
Some slides about the process: https://www.grahamedgecombe.com/talks/minecraft.pdf