Don't confuse the Nikon D5100 auto-focus, with a Sony auto-focus. My NEX 5n, using face detection, does a good job of keeping a person in focus as they walk around a room, even at f/2.8. If Sony can do this, why not Canon
Well, part of it is that they are different technologies. The way a Canon camera autofocuses for pictures is different than how it does in Live View, and video uses the Live View technology. Canon would have to adapt the SLT technology with a translucent mirror to auto-focus in the exact same way a Sony does. In doing so, they'd give up all the photographic advantages of the mirror, which isn't worth it. Basically, the Nikon autofocus sucks because it doesn't use the focus sensor (due to the mirror being up). I doubt Canon will have solved this in a way that makes it comparable with what sony has in their SLT's, let alone what the FS-700 has.
The FS-700's AF is impressive (though he admitted it makes mistakes that he doesn't show), but, it's also technology from a $10k plus camera...I don't expect Canon to implement that in their Rebel line and have skipped the 5DIII with it.