I read his essays on this point but did not understand until this year. He knew that natural selection on groups was a truism. And so he did not even argue at length for it, only focusing on the selection that acts on various sizes of groups, as opposed to accidents of history.
Interactions between organisms that are not zero sum drive the practical importance of natural selection on groups. Species are maintained with a conserved chromosome map and sexual reproduction due in part to the selective incentives for sharing immunological genes.
Michael J. Burns