After looking over the box office returns again, I can't really see any reasonable way to regard decisions on how STID was marketed as mistakes, if by a "mistake" one means something that hurt the bottom line. If STID had been a failure or only a marginal success, then second-guessing or Monday morning quarterbacking the marketing campaign might be in order. But given that STID was a success, how one could validly argue that the returns would have been better, if only someone in marketing had done thus and so, is really a mystery to me.
Likely because as good as STID did, it didn't do as well as many expected it to as a follow up. It did less domestically than the 2009 film, despite having a boost in 3D and IMAX tickets. The fact that Abrams himself regards the marketing strategy in hiding Khan as mistake should tell you something. STID was not a failure, but it's box office run isn't as impressive as other successful franchises and might not be enough for Paramount to want to put out so much money. That's also why there's talks of budgets being lower for the follow up film, because they realize it's not going to make the kind of profit that other franchises like PIRATES, TRANSFORMERS, THE DARK KNIGHT, MARVEL, ect makes.
Would revealing Khan from the get go have changed things? Perhaps, but we'll never really know. Still, it's clearly acknowledged now as a lost opportunity.