I didn't even know there was a Golden Age Psycho-Pirate, but that explains why he's in this .True, we don't even know if the story is set in one year or if it skips around. Like Frontier said, could be a young and old Jay. Or hmm, maybe there's a timeskip coda for the ending with the Flash family. Flash, Iris, and even Aquaman, imo, being in the credits was a definite surprise.
Halstead vs. the JSA were some let's say interesting conflicts in the comics...lol.