When I have seen Seinfeld I noticed that in one of the episodes George Constanza had sung Believe It Or Not from The Greatest American Hero as his answering machine message "you know the one that says Believe It Or George Isn't At Home So Leave Your Message At The Beep" and it makes me think that George was a fan of The Greatest American Hero and possibly Jason Alexander was too. So that's why George sung a parody of Believe Or Not on his answering machine message. That Seinfeld episode makes me giggle every time I hear George's answering machine

David Mandel was the writer for "The Susie" episode.Wish we could ask him. bet you are right.