When I said execs are only meant to know, that doesn't mean they're impossible to leak, it's just if it does that it's through nontraditional channels. For example, Cloud was leaked by Tansut, who just took a friend's words at face value, a few hours beforehand but nobody believed him until after the fact. Ken, Inceniroar, and PP were all technically leaked by a play tester. There could be a legit leak out there and, if history has told us anything, it's probably from a super low level employee who was in the right place at the right time.
Meanwhile, there are a number secrets that Sakurai was able to keep totally leak free. K. Rool and Joker eluded everyone. Robin in Smash 4 and "Everyone is Here" in Ultimate was a surprise to even people in the know. When Sakurai's deadset on keeping something secret, he's pretty good at it. Again, keep in mind that Sabi's source has known all the contents of the Directs for the past year (aside from K. Rool, I believe) but doesn't know what's in this presentation; that's pretty telling, in my opinion.