If that was the real sky, then congrats! If it was fake, then nevermind

No, he made the server available in offline mode, which can be done in a normal non-bukkit server, which allows cracked players who didn't but the game to join. You can log in as sky without a password and join this "offline server" and you will have sky's skin and cape.
Therefore, this is fake.

NPC, or mob player disguise.

its fake most likily