Orochimaru naturally knew his true identity, but being overly familiar with such matters would only be counterproductive.
With how recognizable his outfit was, a bit of intelligence-gathering in the Northern Lands would easily connect him to that identity. If he acted like he knew everything about everyone, Obito would keep a dead lock on him in the future. He wasn't really afraid, just annoyed by the trouble; what he needed right now was time.
"Heh... I have to say, this is the first time I've ever met someone even more arrogant than me, Uchiha Madara!"
Chapter 213