컴포넌트를 다 만들었으면 ArticleScreen을 열어서 기존의 ListHeaderComponent를 <></>로 감싼 뒤, ArticleView 하단에 방금 만든 CommentInput 컴포넌트를 보여주세요.

screens/ArticleScreen.tsx

(...)
import CommentInput from '../components/CommentInput';

function ArticleScreen() {
  (...)
  return (
    <FlatList
      (...)
      ListHeaderComponent={
        <>
          <ArticleView
            title={title}
            body={body}
            publishedAt={published_at}
            username={user.username}
            id={id}
            isMyArticle={isMyArticle}
          />
          <CommentInput articleId={id} />
         </>
       }
    />
  );
}

(...)