Width REQUIRED

I get this message: if ( ! isset( $content_width ) ) $content_width = 900;

Isn’t this the same?

function themename_content_width() {
	$GLOBALS['content_width'] = apply_filters( 'themename_content_width', 900 );
}
add_action( 'after_setup_theme', 'themename_content_width', 0 );

https://www.ads-software.com/plugins/theme-check/