Harnack read Marcion through Luther, and so did the critics who attacked him for it. These three chapters follow that lens where it clarifies and where it distorts: the law-and-grace frame he could not set down, the question of whether Marcion taught two Gods or one antithesis, and the strange fact that the Creator is called righteous and troublesome rather than bad.
This is the longest part of the afterword because it is where the real disagreement sits. The claim throughout is that the antithesis of law and grace is a consequence of the Marcionite confession and not its foundation.