I can only imagine how people would rate Calzaghe if we only had footage like this from him: http://www.youtube.com/watch?v=yU3BpXNCyL4 In my opinion it's better to not predict anything at all because there is in fact no footage of Greb's fights. A short clip of him clowning around with an old timer hardly proves anything either way, unless you want to believe that the man had no ability. In that case he must have had a lot of luck beating proven great boxers such as Tunney, Loughran, Walker and the Gibbons brothers.
Similar styles, but Greb has the best resume in the history of boxing. He was the only man to beat Gene Tunney, and would probably be the only man to beat Calzaghe.
If you were going to go with your eyes then you would decline to draw any conclusion one way or the other because you don't have any footage of Greb. You can't draw any firm conclusions about his style from training footage.
That's another way of angling it, though I prefer rankling the old timers with some Greb-bashing. Of course, Greb was an amazing fighter but the footage of him training is an unfortunate choice of artifacts, simply amateurish.