Well, they weren't active back then, except for maybe Doc(tor Occult) & Rose( Psychic), who were probably a bit obscure. The Phantom Stranger might not have even been on Earth at that time, & if we was may have been still doing the Doctor Thirteen riff of exposing fraudulent magicians.
But even if there had been other public superheroes in the USA, Superboy was the only one in Smallville, & was a dead ringer for the Kents' son. Who at that time was on the books as adopted.
Even in such a face-blind era as Silver Age DC, where apparent doppelgangers popped up like mushrooms, such a coincidence would have exploded S-boy's secret ID (which of course only existed because of such faceblindness/facesameness).