"In the 19th century in America, middle-class girls and women played piano far more than men. Yet all that piano playing failed to result in any creative output."
The sub-point he was making that aptitude or talent does not imply creative output. So he gave an example of a group that had extremely high ability in an area but did not produce in that area. With the bigger point being that differences in achievement in certain areas between women and men may come down to interest more than ability.
The logical counter-response to this would be "well there were so many social barriers (de-facto and de-jure) preventing women from producing creative output"
He then gives an example of a similarly or more repressed group with extreme ability in an area (Black jazz musicians) that managed to be extremely prolific in that area.
It is actually pretty convincing. Or at least thought-provoking.
Well, I'm convinced!