The Admiral said she didn't even recognize who he was anymore. He knew exactly how to get to the parallel universe. He was acting like a mirror universe character the whole time, being nasty and ruthless in a way we've really never seen, gritty Star Trek or not. I didn't really feel like Lorca being the mirror Lorca was a shock.
Yeah, it was certainly well foreshadowed, but that's not the same as it being expected.
I mean c'mon, who expected a Mirror Universe episode so early, let alone it being all a major arc from the get go. It's one of those great reveals that make you go "of course, makes perfect sense", while not actually seeing it coming.
...
Also, it just dawned on me that, with his access, he might know about the Klingon War and how it ended, based on the data they retrieved from the Defiant, which only disappearance after, so there's a lot of room for them to take the story in... Hell, even the Emperor knowing stuff would work.