Apparently you've never seen an E3 demo where the actor screws up but somehow the demo magically keeps going.
I get what you're saying, but these guys are the master of deception. Even motion control games are faked. It looks really, really bad for them to have something unscripted (in the "this wasn't in yesterday's practice session" sense) happen.
But it does happen, and has happened often.
Not everyone will actually attempt to dupe the audience. Rather, they'll actually put time and effort into preparing for the demo - often someone playing is someone who has worked in depth with the title, and likely coached for or practiced enough to know the best "tricks" to showcase things with the right timing to not make any particular "issue" stand out.
It could be faked, but nothing about it screamed "this just isn't right."
It really stands up to the Naughty Dog style thus far too - you walk into "scenes" ... certain things are going to happen, but individual characters may behavior different on each play-through. If you've played enough Uncharted, you can see how that demo very much plays out like a live event.
Just watch the character movement and reactions - the player wasn't even sure where certain enemies would be.
Sony has had enough "oops" moments during conferences that I'm pretty sure they just expect a damned good effort for live presentations, but have accepted the occasional issue.
Again, it could very well be entirely scripted and just well-acted on stage... but imho, if so, they went out of their way, way out, just to make it appear live.
Not that I care because the game will play just like that in reality. ND has shown themselves to be entirely capable of such a product.