He's saying that if the economic gains where wiped out, then blacks and whites should have had equal economic standing after the war. But we all know that didn't happen, which is why he posed that question.
Even if it was the case that the civil war wiped out the gains from slavery, it doesn't change the case for reparations as well as the other injustices that were being committed are are still being committed to this day
That’s almost certainly not the case.
Any economic gains from slavery were more than wiped out by the civil war.