The video did make some fans did wonder if Mirror's Edge was originally intended to be in third-person, asking why the game would even have a full body model if it's in first-person. Her movements were awkward, and the third-person run-through is just a rough ride.
Video is still up; see for yourselves!
So what's the deal? Tom Farrer clears things up in a brief Q&A session with the folks at 1UP:
On whether the developers intended for players to find this:
"Mirror's Edge has always been a first-person game and it was never designed or intended to be a third-person one. It doesn't really surprise me though. The PC community always has a lot of fun playing around with games, finding ways to modify and experiment with them. It's always fun and interesting to see what they come up with."
On why Faith's model looks better than you might expect:
"What you are seeing is the shadow mesh. Faith has a basic third-person rig and animations that are used to cast her shadow. If we cast a shadow from the first-person rig it would look very odd indeed so that's why it's in there."
On whether he ever doubted that first-person was the way to go:
"No, we've never doubted that it was the right thing to do. The first-person viewpoint is what makes the game unique and gives the experience so much power. There's always a degree of separation when playing in third-person. We didn't want you to watch her jumping between rooftops; we wanted you to feel what it was like to actually do it. At no point in production did we ever consider going third-person. There are already great third-person movement games out there. Our goal was to create a new experience for players."
Consider this mystery solved.