That's only true to a point. Sure, socialization is a big part of forming our identities but so is our own investigation into our ancestry. I think it's entirely possible for children of immigrants to grow up in one culture and then in the process of maturing, expose themselves to the history and culture of their parent's in any way, shape, or form. Heritage IS a large part of our identity as long as you understand the culture of that heritage. It becomes part of your cultural identity.
Just as different people will encounter Eastern philosophy and decide to educate themselves on Taoism and accept Taoist idealogies. They do not necessarily have to be exposed to a Taoist society but they can learn about it and accept the culture of it.
As far as it applies to Superman, it can be assumed that the systems in the Fortress has educated Clark on Kryptonian culture and history. Depending on Clark's acceptance of those ideologies, he could adopt Kryptonian culture into his identity.