Personally I think that "I'm Superman" is the perfect opener to a more in depth explanation. It sums up completely the fact that Clark is an alien with superpowers and he spends he free time saving the world. I don't think it takes away from the reality that 'Clark' is the real person at all. Clark thinks of Superman as a job or it can even be equated to a actor playing a role on TV. Two nights ago my husband finished watching Defiance Season 2 and then he went on to watch the specials/behind the scenes. One of them was about the makeup being applied to a particular character. The actor happened to mention that when he tells people he is on Defiance they don't believe him, because he is in so much makeup. So, if he was to say to someone "I'm Alak Tarr", does it take away from his real identity? No, it just explains what and who the role is that he is playing.
Now, if Superman didn't exist, then that is a different scenario entirely.
