They said in the keynote that stylus input can bet determined separately from other screen input so if the stylus is in use, the app would be able to ignore other screen inputs. This to me says palm rejection.
Disclosure: my team was one of those present at the event.
The APIs allow you to check the touch type (direct = finger, stylus = pencil). This is kinda key for some situations like in Paper, where finger does one action, while the pencil does another. Failed palm rejection in this case might not get picked up by a gesture at all, and you are right that apps can shut everything off but the pen in certain cases to ensure things work. The problem with current styluses is that palm rejection in an app is looking at a bunch of "identical" touches from the OS and trying to decide which is which. Here you don't have to guess what is coming from the pencil, and the OS can reject large touch surfaces at the driver level that apps can only guess at.
That said, it does have palm rejection at the OS level, but since we only have a beta build of 9.1, I'm hesitant to say anything definitive about how well it works beyond that it does beat all the stylii I've tried to use so far on the iPad, and not by a small margin. But to comment on the angles people used in the video: note how the artist can see what they are sketching easier depending on their hand position. Coming in from the top makes certain parts of the "page" easier to see. Both for the artist, and the camera recording it. My recommendation is to try it in the store if you have doubts.
Me? I'm getting one based on my time with it. This is the stylus on the iPad I've been trying to find for years, and haven't found until now. And the larger screen is actually something I've got uses in mind for (split screen PDF/whatever and notes app? Yes please). But I might find it doesn't entirely replace my Air to use before going to bed, we'll need to see there.
The Adonit styli are far from fine. They are ok in certain situations, but are not really that accurate or reliable. I will be very surprised if Apple hasn't integrated palm rejection somehow into this, but on the other hand, I know from experience that palm rejection that tends to work ok for drawing or marking up documents as they demonstrated often doesn't work if you are trying to take handwritten notes, something they didn't really make a big deal of.
Yeah, don't get me started on the Adonit stuff. My current "favorites" are 53's Pencil (which still has issues), and the AluPen (doesn't try to be something it can't). But this sucker is something else.