[Sakurai] caught wind of the leak, and how people were getting totally unsurprised by his reveals... so he basically made it very explicit how he was going to reveal Shulk today, telling Bandai Namco as much, and even getting a Monolithsoft employee to retweet the announcement. This made the leaker extremely confident in saying "it's gonna be Shulk

" to Sal. However, Sakurai kept the truth close to his chest - that he was revealing Lucina/Robin today - and all of this was A) an attempt to surprise us once more (he succeeded, that's for sure), and B) an attempt to smoke out who the leaker is so their ass can be shown the door.
(This probably isn't the truth, but it makes for interesting reading.)