Honestly the most impressive part to me was being able to convey a story of "human somehow put into a machine" pretty much only through physical acting. That's not something you see every day in video games.
And I suppose it's worth pointing out that creating all the tooling to put this together is just as impressive as a lot of the more obvious aspects. It's a giant content creation pipeline coming together. I expect these demos from Epic, not Unity. They're stepping up.
I mean, anyone can come up with some convoluted ideas that explain why the above does make sense... like the robot breaths because there's an organic brain in there that needs oxygen, they moan because it's still a human brain and the machine is sending his brain an overload of sensory data that is hard to deal with, the robot stumbles because his brain is new to interfacing with its machine parts etc. But I personally didn't like it and kinda roll my eyes when they go overboard with the anthropomorphism. Still it was hella cool, hope the full version will explain away my doubts neatly :)
To be clear though, I think your opinion on this is every bit as valid as mine, I just really enjoy discussing film.