Christopher Reeve said this because of the extraordinarily innate abilities found in superman. Superman's extraordinary abilities have been displayed from birth as against other superheroes who become extraordinary in their aging process. "American heroes are usually rugged individuals linked to the Horatio Alger archetype - plain folks from humble but often rural beginnings who react gracefully to the hands they are dealt or achieve the impossible against the odds"
Other heroes have several vulnerabilities including self-doubt, but superman has just one vulnerability- a green rock.
He said this because being the actor that played Superman, after his injury leaving him paralyzed, he continued to play the "superman" by helping himself out and helping other people with injuries similar to his. He also noticed the natural abilities Superman had and were born with. Other heroes weren't born great they had to be developed, but Superman, he was born already a natural.
An allusion in a play could be referring to another play, another piece of writing, an event in history, or there are biblical allusions where they refer to stories and event within religion.
