Which is exactly my point. What is he playing as if there is no Superman figure for him to imitate? In a world without superheroes with capes, where do the kids get the idea?
Probably the same place Siegel and Shuster got the idea. Superman's costume wasn't pulled out of the ether, it was based on the costumes that circus strongmen wore in the 1930s. Maybe in a world where that look didn't become associated with superheroes, it continued to be associated with circus performers. For that matter, Evel Knievel wore a cape during his career as a daredevil/stuntman, so even in real life the custom lasted at least until the '70s. Young Clark could've been emulating his favorite daredevil or acrobat or weightlifter.
Plus, maybe Clark was a Zorro fan?
And I seem to recall Flash Gordon sporting a cape occasionally.