Know what? They've specifically stated in interviews its been development 14 months and in others said they're intending to show it in the next couple of months (I have a hard time believing they aren't referring to Gamescom, it would just be silly if they weren't)
The rest of it, well a two punch PR win is just common sense because of its (current) release date: Announcement that people are intrigued by -> the talent behind it and the concept itself and its out relatively soon. Then you take it to Gamescom, show gameplay -> (presumably) hype intensifies, people are satisfied due to short lead time between announced, gameplay and release.
You make the most of your announcements and unveils, you don't force something that doesn't make sense. CGI trailer is perfectly fine for me, I know this game exists and I'm excited about it as others are, we get to see it soon according to the developer, win for gamer, win for publisher, win for developer.