It absolutely is about the hardware (and what they've done at the OS level to support that). The resolution and extremely low latency of the stylus makes this a different class of experience - akin to the conceptual break between old touchscreens and modern 1:1 instantaneous response (iPhone, Android 4.1+, etc). It's a distinction lots of people overlooked at the time, but I'm willing to bet it will be as fundamental this time round as it was with touch. I've had all three Surface Pros, each time hoping the stylus input would be good enough for creative work, and it just isn't. It's still just the basic input method that Jobs dissed. If the iPad Pro does what it appears to from the videos, this is conceptually different from any other current device.
Though I'm still hoping the Surface 4 has gone the same route...