It does make sense that Manning would be working out at Duke because that is where his former offensive coordinator from UT, David Cutcliffe, is the head coach. Cutcliffe is a good friend of the Manning family, and he was Eli Manning's head coach at Ole Miss.
There was a time a few months ago that I was convinced that Manning would never play again. While this video really doesn't give us enough to say how close he is to being ready, if this is even Peyton, it does tell us that whoever that is throwing looks to be able to "make all the throws."
It is going to be interesting to watch this all play out.