How to show first name and last name in custom post type columns

The current code calls get_the_author, so it’s showing you your users’ configured display names, not their usernames specifically. One fix is to change all the display names to First Last.

Alternatively, you’ll need to write a new column handler. By default it checks for methods on the list table class called _column_<name> or column_<name> to call, and if it doesn’t find one it tries an action:

/**
 * Fires for each custom column of a specific post type in the Posts list table.
 *
 * The dynamic portion of the hook name, `$post->post_type`, refers to the post type.
 *
 * @since 3.1.0
 *
 * @param string $column_name The name of the column to display.
 * @param int    $post_id     The current post ID.
 */
do_action( "manage_{$post->post_type}_posts_custom_column", $column_name, $post->ID );

so you need define a handler e.g.

function manage_stores_custom_column_handler( $column_name, $post_id ) {
    if ( $column_name == 'author_fullname' ) {
        $post = get_post( $post_id );
        if ( $post && $post->post_author ) {
            $author = get_userdata( $post->post_author );
        if ($author->first_name && $author->last_name ) {
            echo $author->first_name . ' ' . $author->last_name;
        } else {
            // Fall back to display name
            echo $author->display_name;
        }
        return;
    }
}
add_action( 'manage_stories_posts_custom_column',
            'manage_stores_custom_column_handler', 10, 2 );

Note that this column won’t be linked as the original column is: unfortunately the code that does that is in a protected method in WP_Posts_List_Table so you can’t just call it but you could copy it out into your own code if you needed it.