The director believed that Truman would never guess the artificiality of his world and life that Truman believed in the reality of the only world he had ever known.Do you think this statement is true?

Most people believe in the reality of the only world they have ever known, so there is no particular reason to think that a person raised in an artificial world would be any different, as long as he had no other information to work on.

Note that he adjusted his reality when he discovered things that it did not cover. It's a shame real humans aren't as good at that sort of thing.