Telling the public why the products are great is much different from spinning why the products need to be thought of as great. The iPhone 4 leak may have taken the "wow factor" away from Steve Jobs' keynote debut, but I don't think it had any kind of harmful effect on perception or sales of one of the greatest devices ever to come out of Cupertino.
On the other hand, revealing Animoji as the prime feature of the iPhone X with iOS 11 gave enough of a head start for the public to realize it was a letdown and collectively ask "is that it?" even before the reveal.
I understand wanting to keep as far ahead of competitors as possible, but it seems like Apple is more concerned about suppressing negative perceptions rather than protecting the secrecy of genuinely revolutionary new products.
It comes down to stark differences. Jobs was more of a visionary than Cook. Cook is more about maximizing returns.