I do take your point that pretty much everyone who will see MOS probably knows Superman's origins.
I work in a museum where I regularly use a Superman analogy to discuss the iconography of Greek gods and goddesses in ancient works of art. To explain how, say, Athena, is easily recognizable to an ancient Greek seeing her image on a vase painting, or statue because of her war helmet, shield, spear and Medusa's head on her breastplate, I ask people, "If you walked into a room and saw a large staute of a muscular man in blue tights, red boots and red cape and a big S on his chest - who is that?" Always, at least 3/4 of the crowd says Superman. (I have done this with literally thousands of people.) I then say - now, you not only know his name but you probably know what he can do - fly, leap tall buildings, faster than a speeding bullet. Again, about 3/4 of the crowd will nod. Then I say, you may even know some of his story - where he's from, what his day job is, who his girlfriend is.* Maybe 1/4 to 1/3 of people tend to respond to this.
My point is - I don't think everyone does know Superman's origin. I think almost everyone can recognize Superman, and has some notion of him - but a lot of people's notions are pretty vague.