If it just turns out to be, "OMG, he's Khan," then big deal, because whether he's Harrison or Khan, he's obviously a badass and extremely dangerous. Whoever he is, he looks like he'd bitch-slap the Khan from the prime universe and ask him to make him a cup of tea.
As far as I'm concerned at this point, if he's Khan, then there's going to have to be some other big surprising payoff from him being Khan to have made the secrecy (or at least continued denials) make sense. Everyone in the new universe is meeting him for the first time. They know nothing of "Space Seed" or TWOK, so the secrecy behind his real identity has to fit into the story, I hope, and also shock our heroes for some reason. (And I'm assuming it will.)