It's impressive because it's most likely fake.
I didn't see anything in the video that looks particularly difficult to achieve in a modern engine. The volumetric fog and clouds are really nice and might be difficult but I'd be surprised if they weren't doable.
Everything else just looks like good models, textures, and lighting to me. I don't even see any particularly difficult techniques.
And there are things that definitely scream "real-time" to me. In particular, the shot of the jet taxi-ing looks like it might have some poorly tuned screen space global illumination around the jet. Notice that fake looking soft shadows around the engines that move with the plane.
Obviously, the whole world isn't going to be modeled and textured with the level of fidelity you see here. The trailer is only going to show the "hero" locations that get a lot of care and attention — Seattle's space needle, the pyramids of Egypt, etc. But I see no reason why the in-game experience wouldn't look like this in those areas.
Strictly speaking, this is true, but note that using modern datasets (like the ones used by Google/Bing Maps with the extensive lidar/photogrammetric/whatever 3D data) and modern internet speeds and infrastructure for streaming map data (again like what's used for Google/Bing Maps/Earth) will mean that the filled in areas are far more expensive and detailed than in flight sims of the past that had to assemble their own scenery and distribute it themselves via one-time download or physical media
For some idea of how amazing this will be, check out Google Earth VR.
Another thing that I noticed was some rock clusters that accidentally covered roads on the terrain, which makes me think there will be a lot of proceduraly generated content to add details into to the big open areas.
Those two things combined I believe will make it possible to do the entire globe to levels of fidelity that will be incredible to fly in. Some areas might be a bit more "imagination" than reality, but hundreds if not thousands of cities have really great lidar scans they could incorporate. It's going to be an insane leap forward, and as they're scans it will be like flying over the real thing. In theory, I could fly over my own house in the city and actually see it.
I guess we'll see what kinds of concessions they've had to make to fit it into a realistic game package, but I'm pretty hopeful as is probably evident.
If you look closely, some scenes are photo real. Other scenes, like the giraffes and a few frames before that, seem to be in the uncanny valley.
I suspect that this game may be this generations Crysis. It requires ungodly horsepower to render at full settings, and perhaps a more-than-robust internet connection to transfer all the imagery data.
The same for X-Plane 11. You can get some decent performance if you let run separate process for each monitor though.
Both indicate to me that the problem lies in software. They also have in common that performance gets abysmal when you add more moving objects.