See, that's the thing though. I'm fully aware of how seeds work (kind of a requirement for a computer science degree) and they do not change the rules that govern how a world generates, only the 'start point' of the world generation. What were seeing in the 'hands on' gameplay doesn't appear to be just a similar seed, and is instead more akin to the special world types you see in Minecraft.
Now if this 'world type' for the demo just so happens to turn the random encounter rate for wildlife, structures, etc. wayyyy up, then ok. But from everything I see it seems much more likely, as it is common practice within the industry, that is just scripted. If that is the case then it wouldn't be a big surprise, and I'm not even saying it is a bad or shady thing to do. But why would they claim its true gameplay footage of what the full game experience will be like if it isn't?