I remember some of the older ones, I think it was Black Flag that was leaked by some random guy that was apparently sitting behind a Ubi employee in a train/plane while he was working on some powerpoint presentations for it and he just told people everything he saw. Sounds pretty bizarre for sure. But everything turned out to be true.
That was Syndicate. Yes, the leaked info turned out to be accurate. I can't recall a single major AC leak that
didn't turn out to be accurate. Closest thing would be that AC "Empire" was going to be "set in Egypt, Greece, and Rome," but it turned out to be AC Origins which was set in Egypt at a time when Greeks and Romans had imported their culture there.
For me a big clue is that Ubisoft joked about all of the AC leaks in Watch Dogs 2. If the leaks were legit (i.e. unwanted, embarrassing, and most of all ILLEGAL,) I really doubt Ubi's Business Affairs (Legal) dept. would be cool with them being talked/joked about in games. But if the leaks were engineered by Ubi, that's a different story.
A professional game designer just up and out of nowhere saying to a stranger sitting next to him "Hey, you like video games? Want to see something really neat?" and opening his laptop... sure, it's possible. But imo it sounds like something a not particularly imaginative 'viral marketing' monkey would come up with.