I'm not missing the subtext at all. The fact that writers previously decided to write these characters with a specific market in mind is one thing, but the potential and the nature of these characters present something far greater.
Superman is the ultimate story of an immigrant, but he's an immigrant in this world. America was the alley where he grew up. For example in Returns Lois writes about "Why the world doesn't need Superman". The world, not america. His values are human values, they aren't american values. His reach is worldwide, he's not Spiderman who is basically just making a difference in New York.
Lex represents human flaws, nothing he is is exclusive to him being american. He also doesn't characterize the american dream at all considering he was born rich and the American dream is about social mobility. Rather he contrasts by being a human with power who corrupts his own humanity, the opposite of Superman who isn't even a human, and his power made him the most humane of all of us on Earth. That's why he is a worldwide symbol, why his message is far reaching.
It's not about Americans. That's just sugar coating, because he's written by Americans.