Vonnegut's statement that he is a "real person" could mean that he wants to be seen as a genuine and authentic individual, rather than a fictional or fabricated character. He may also be emphasizing his own humanity and the importance of personal experiences in shaping one's worldview and perspectives.
He may be working hard to convey this point because he wants to stress the importance of personal authenticity and individuality in literature, and the power that real human experiences can have in shaping the stories we tell. He may also be pushing back against the idea that authors should be detached from their own experiences, and that the literary world should be separate from the real world. He may want to show that authors are real people with real experiences that shape their work.