looking at MySQL query in the function it becomes clear that though it matches the post_name and returns true or false. What you want to find is, if a page exists by its slug. So, in that case, the function you have chosen is wrong.
To get the page by title you can simply use:
$page = get_page_by_title( 'Sample Page' );
echo $page->title
Just in case if you want to get page by slug you can use following code:
function get_page_title_for_slug($page_slug) {
$page = get_page_by_path( $page_slug , OBJECT );
if ( isset($page) )
return $page->post_title;
else
return "Match not found";
}
You may call above function like below:
echo "<h1>" . get_page_title_for_slug("sample-page") . "</h1>";
Let me know if that helps.