is_front_page() is only for use on the front-end to tell if the ‘main query’ is for the front page. In a back end context you need to check if the current post ID (which is in the $post passed to your callback function) is the same ID that’s set to be the front page.
So the if statement would look like this:
if ( $post->ID == get_option( 'page_on_front' ) ) :
endif;