This appears to be late in the JSA's history (it was only 10 years ago not in the Golden Age and the wording implied the JSA had been around a while and had a legacy), and it mentions Sylvester being the Star Spangled Kid before getting the rod/staff and becoming Starman, so I am assuming it is referencing/channeling the stories where Knight gave the Rod to Pemberton (which Sly then improved into the staff) and hoped Sly would be his heir since neither son was interested. That's a lot of convoluted backstory to include without much pay off to Courtney's story though, so the shorthand is to just make it Sly as Starman after being the Kid and let comics fans fill in the other backstory for themselves while the general audience doesn't need it to enjoy the show itself. Sly embodies the entirety of that story for comic fans but also presents a more accessible entry for non-fans.
-M