Well, it's true. They did develop skills under slavery that could be applied to their benefit.
That doesn't mean slavery was a good thing for blacks to go through, nor does it imply that it was. That's just a recognition of reality.
I don't think it is the right approach to teaching the history of slavery in the US for several reasons, mainly because pointing that out is kind of like: "Duh, no shit"...but it is true that many slaves, if not all, learned skills that proved beneficial to them once slavery was ended.